The challenge: The shortest code, by character count, that detects and removes duplicate characters in a String. Removal includes ALL instances of the duplicated character (so if you find 3 n's, all three have to go), and original character order needs to be preserved.

Example Input 1:

Example Output 1:

Example Input 2:

Example Output 2:

(the second example removes letters that occur three times; some solutions have failed to account for this)

(This is based on my other question where I needed the fastest way to do this in C#, but I think it makes good Code Golf across languages.)

+7  A: 


new string(input.GroupBy(c => c).Where(g => g.Count() == 1).ToArray());

71 characters

Bryan Watts
Found out it could actually be even a few chars less with your method, changing cs to c and removing whitespace. Then it becomes 66 chars. Though I've found a method one char less.
"input" is string, so it doesn't have GroupBy method. You should add call to "ToCharArray" method.
System.String implements IEnumerable<char>, so it has the GroupBy method. The IDE makes an exception for the extension methods on string and doesn't show them, but they compile and run just fine.
Bryan Watts
Didn't know that, thanks
Bryan, that's only the case for Visual Studio, as I did see them appear in #D.
Fair enough, sometimes you forget there are alternatives :-)
Bryan Watts
+15  A: 


print filter(lambda c:s.count(c)<2,s)

This is a complete working program, reading from and writing to the console. The one-liner version can be directly used from the command line

python -c 's=raw_input();print filter(lambda c:s.count(c)<2,s)'
That doesn't seem to work in my version of Python unless i add square brackets around the array constructs - join([...]).
That's a generator expression and was added in Python 2.2. I think it's reasonable to assume everyone who actually uses Python is on that version or higher.
I'm using Python 2.3.4 (RHEL4), and it moans about the above expression - "SyntaxError: invalid syntax". Only in Python 2.4 and above perhaps?
Generator expressions were added in 2.4. Anyway the new version is shorter and doesn't use them.
Steve Losh
You can just use `input()` to reduce char. And can you please add a char count?
@vlad003: Not in python 2.x
Yes. I forgot it doesn't work with strings.
+25  A: 


21 characters of perl, 31 to invoke, 36 total keystrokes (counting shift and final return):

perl -pe's/$1//gwhile/(.).*\1/'
William Pursell
It's customary to post the language and the character count.
Chris Lutz
(Disclaimer: I'm no Perl guru.) I tested with v5.8.8 and it seems you can remove the space after -pe to save a character.
There are a few characters we can remove. You are correct about that space. Also, the space after the `while` should be superfluous and the semicolon is also superfluous since it's only one line. However, 35 characters is in _no_ danger of being beaten any time soon.
Chris Lutz
The apl solution of 23 characters requires that the excess spaces be removed.
William Pursell
Damn, I didn't know Perl would let you get away with `s///gwhile`. That's kind of ridiculous.
Chris Lutz
`perl -pe'y///cs'` would work, **if** the copied characters were guaranteed to be next to each other.
Brad Gilbert
+9  A: 

C89 (106 characters)

This one uses a completely different method than my original answer. Interestingly, after writing it and then looking at another answer, I saw the methods were very similar. Credits to caf for coming up with this method before me.


On one line, it's 58+48 = 106 bytes.

C89 (173 characters)

This was my original answer. As said in the comments, it doesn't work too well...


On two lines, it's 17+1+78+77 = 173 bytes.

+1 for using one of the more difficult languages
You're calling `realloc` and `memchr` without compatible declarations in scope.
What language?
@Svish - C. Perhaps you should get out more. I don't know Ruby or C# but I can more or less recognize them when I see them.
Chris Lutz
@caf: Not true. Calling realloc() on NULL (b is initialized from 0) is fine, and acts like malloc().
@unwind - He's saying that they're not declared, i.e. no `extern void *realloc(void *ptr, size_t len);`. Of course, all you really need is `void *realloc();`, but they _do_ technically need to be declared since they don't return `int`.
Chris Lutz
int is just casted to char *, which I take for granted. Yes, it's non-standard, but the necessary #include's brings up the file size too much. Maybe I can trim it down even more or even not use memchr.
It's not cast, because the compiler doesn't even know that those functions are returning `void *` (eg imagine what would happen under an ABI that says pointer and scalar return values are returned in two different registers?)
On your new one - you're assuming that `EOF` is negative (that's not actually guaranteed - it could be a positive integer outside the range of `unsigned char`). By the way, the definition of `EOF` is the only reason our solutions need `stdio.h` - the implicit declarations of `getchar()` and `putchar()` are compatible (unlike `memchr` and `realloc`). And remember that objects with static storage duration are guaranteed initialised to zero. (I'm also not completely comfortable that our solutions fail to `return` a value from `main`).
@strager - You don't strictly need the headers. You know the definitions, you could just say `void*realloc();` and it would be declared.
Chris Lutz
@caf, I see your point about ABI's and such. Also, everywhere I look I see EOF must be a negative integer. @Lutz, I see.
Sorry, you're right that EOF must be a negative integral expression.

I couldn't find the original answer, but in the spirit of a Jon Skeet answer:



The meaning of this program is: Read a string from standard input (R), Strip all duplicate characters (S), Print the result to stdout (P).

Martin v. Löwis
Skeet would have done it in one character. :-P
Joel Potter
Martin _did_ do it in one character - but he then took on the harder challenge of writing it as a full program instead of just a function.
Chris Lutz
-1 for not being Jon Skeet and I wish I could add another -1 for annoying fanboyism.
How about -1 for having fun while you're at it?
Chris Lutz
Ha ha, yes, let's reuse Jon Skeet's jokes. It was amusing when he did it, but now it is just tired.
I'm not saying it's a paragon of comedic genius, but it made me chuckle. If you're tired of it, that's fine - just don't read it.
Chris Lutz
+1 for karmic neutrality.
David Thomas
+3  A: 

Javascript 1.8

s.split('').filter(function (o,i,a) a.filter(function(p) o===p).length <2 ).join('');

or alternately- similar to the python example:

[s[c] for (c in s) if (s.split("").filter(function(p) s[c]===p).length <2)].join('');
Why you keep all the insignificant whitespace characters?
@KennyTM didn't realize I could remove them?
@Breton: Like `[s[c]for(c in s)if(s.split("").filter(function(p)s[c]===p).length<2)].join('');` so the length is reduced to 79.
+4  A: 


For Each c In s : s = IIf(s.LastIndexOf(c) <> s.IndexOf(c), s.Replace(CStr(c), Nothing), s) : Next

Granted, VB is not the optimal language to try to save characters, but the line comes out to 98 characters.

I'm no VB expert, but it looks like there's a bunch of Whitespace in your answer, Does vb require all that whitespace?
If it is anything like VB6 then it forces the whitespace on you.
Yeah, the IDE adds the spaces for you.
No, most whitespaces are absolutely useless here. Being an advanced IDE, VS inserts them for you. But in a code golf they’re shouldn’t count.
Konrad Rudolph
Also, you can replace `IIf` by plain `If`. Better code, less characters. And just porting the C# solution from below would have been shorter still.
Konrad Rudolph
Yes, it will work without the spaces even if VS automatically inserts them. And using `If` instead of `IIf` makes it better. You can also skip converting c to string. And why not replace with `""` instead of `Nothing`... All of these will result in this 75 char code: `For Each c In s:s=If(s.LastIndexOf(c)<>s.IndexOf(c),s.Replace(c,""),s):Next`
You can turn off the VB Specific "Pretty listing (reformatting of code)".
+12  A: 


There's surely shorter ways to do this in Haskell, but:

Prelude Data.List> let h y=[x|x<-y,(<2).length$filter(==x)y]
Prelude Data.List> h "nbHHkRvrXbvkn"

Ignoring the let, since it's only required for function declarations in GHCi, we have h y=[x|x<-y,(<2).length$filter(==x)y], which is 37 characters (this ties the current "core" Python of "".join(c for c in s if s.count(c)<2), and it's virtually the same code anyway).

If you want to make a whole program out of it,

h y=[x|x<-y,(<2).length$filter(==x)y]
main=interact h

$ echo "nbHHkRvrXbvkn" | runghc tmp.hs

$ wc -c tmp.hs
54 tmp.hs

Or we can knock off one character this way:


$ echo "nbHHkRvrXbvkn" | runghc tmp2.hs

$ wc -c tmp2.hs
53 tmp2.hs

It operates on all of stdin, not line-by-line, but that seems acceptable IMO.

Mark Rushakoff
For those interested I've posted a haskell solution in a pointfree style: http://stackoverflow.com/questions/1344352/code-golf-duplicate-character-removal-in-string/2238192#2238192
Geoff Reedy
+3  A: 


123 chars. It might be possible to get it shorter, but this is good enough for me.

proc h {i {r {}}} {foreach c [split $i {}] {if {[llength [split $i $c]]==2} {set r $r$c}}
return $r}
puts [h [gets stdin]]
+3  A: 


63 chars.

puts (t=gets.split(//)).map{|i|t.count(i)>1?nil:i}.compact.join
unfortunately you can't call `count` on an array, you must call it on a string (I made the same mistake >.<)
In at least 1.8.7, Array.count: http://www.ruby-doc.org/core-1.8.7/classes/Array.html#M000333
aah, guess I should upgrade. :P
+3  A: 


96 characters for complete working statement

Dim p=New String((From c In"nbHHkRvrXbvkn"Group c By c Into i=Count Where i=1 Select c).ToArray)

Complete working statement, with original string and the VB Specific "Pretty listing (reformatting of code" turned off, at 96 characters, non-working statement without original string at 84 characters.

(Please make sure your code works before answering. Thank you.)

+3  A: 


54 chars for the method body only, 66 with (statically typed) method declaration:

def s(s:String)=(""/:s)((a,b)=>if(s.filter(c=>c==b).size>1)a else a+b)
Fabian Steeg
+3  A: 


Full program in C, 141 bytes (counting newlines).

Can you declare variables without types in ANSI C anymore?
Chris Lutz
Nevermind. Apparently you can, though GCC gives a warning even with all warnings off about this.
Chris Lutz
Very nice! I'll have to optimize my own method now to beat yours. =]
I managed to get down to 136 bytes myself, and it happened to use a method similar to yours. (It may be identical, even...)
+8  A: 


65 Characters:

new String(h.Where(x=>h.IndexOf(x)==h.LastIndexOf(x)).ToArray());

67 Characters with reassignment:

h=new String(h.Where(x=>h.IndexOf(x)==h.LastIndexOf(x)).ToArray());
I got it down to 53 characters...
Obvious solution. I feel stupid now :P.
+18  A: 

Ruby — 61 53

61 chars, the ruler says. (Gives me an idea for another code golf...)

  puts ((i=gets.split(''))-i.reject{|c|i.to_s.count(c)<2}).join
||    |    |    |    |    |    |    |    |    |    |    |    |    |    |  |
|0         10        20        30        40        50        60        70 |
|                                                                         |
  gets;{}while $_.gsub!(/(.).*\1/){$&.tr $1,''};puts $_

... 53 (by Nakilon)

With the newer version of ruby, you can drop the '.to_s'. I actually like yours better, for a few different reasons.
like the ruler :)
+5  A: 


61 characters. Where $s="nbHHkRvrXbvkn" and $a is the result.

[email protected]{}

Fully functioning parameterized script:

[email protected]{}
I managed to get it to 58. I can't believe I just spent 20 minutes on this.[Char[]]$s|%{if($s -clike"*$_*$_*"){$s=$s -creplace$_,''}}
Josh Einstein
nicely done....
+6  A: 

PHP (136 characters)

function q($x){return $x<2;}echo implode(array_keys(array_filter(

On one line, it's 5+1+65+65 = 136 bytes. Using PHP 5.3 you could save a few bytes making the function anonymous, but I can't test that now. Perhaps something like:

echo implode(array_keys(array_filter(array_count_values(str_split(
stream_get_contents(STDIN))),function($x){return $x<2;})));

That's 5+1+66+59 = 131 bytes.

+3  A: 


(1st version: 112 characters; 2nd version: 107 characters)



/* #include <stdio.h> */
/* int */ k[256], o[100000], p, c;
/* int */ main(/* void */) {
  while((c=getchar()) != -1/*EOF*/) {
    ++k[o[p++] = /*(unsigned char)*/c];
  for(c=0; c<p; c++) {
    if(k[o[c]] == 1) {
  /* return 0; */

Because getchar() returns int and putchar accepts int, the #include can 'safely' be removed. Without the include, EOF is not defined, so I used -1 instead (and gained a char). This program only works as intended for inputs with less than 100000 characters!

Version 2, with thanks to strager 107 characters

#include <stdio.h>

/* global variables are initialized to 0 */
int char_count[256];                          /* k in the other layout */
int char_order[999999];                       /* o ... */
int char_index;                               /* p  */

int main(int ch_n_loop, char **dummy)         /* c  */
                                              /* variable with 2 uses */

  (void)dummy; /* make warning about unused variable go away */

  while ((ch_n_loop = getchar()) >= 0) /* EOF is, by definition, negative */
    ++char_count[ ( char_order[char_index++] = ch_n_loop ) ];
    /* assignment, and increment, inside the array index */
  /* reuse ch_n_loop */
  for (ch_n_loop = 0; ch_n_loop < char_index; ch_n_loop++) {
    (char_count[char_order[ch_n_loop]] - 1) ? 0 : putchar(char_order[ch_n_loop]);
  return 0;
EOF is not defined to be -1.
I also don't like limiting the input length...
Some suggestions: You can save a byte by putting c as parameters to main. You can also use k[o[c]]-1?0:putchar(o[c]); in your second loop to save a few bytes as well. You can maybe save bytes (in source size and memory) using 9's instead of 0's for your big array.
Now we are tied for 107. =]
+18  A: 


23 characters:


I'm an APL newbie (learned it yesterday), so be kind -- this is certainly not the most efficient way to do it. I'm ashamed I didn't beat Perl by very much.

Then again, maybe it says something when the most natural way for a newbie to solve this problem in APL was still more concise than any other solution in any language so far.

+1 just because it's APL
Mark E
+4  A: 


(just knocking a few characters off Mark Rushakoff's effort, I'd rather it was posted as a comment on his)

h y=[x|x<-y,[_]<-[filter(==x)y]]

which is better Haskell idiom but maybe harder to follow for non-Haskellers than this:

h y=[z|x<-y,[z]<-[filter(==x)y]]

Edit to add an explanation for hiena and others:

I'll assume you understand Mark's version, so I'll just cover the change. Mark's expression:

(<2).length $ filter (==x) y

filters y to get the list of elements that == x, finds the length of that list and makes sure it's less than two. (in fact it must be length one, but ==1 is longer than <2 ) My version:

[z] <- [filter(==x)y]

does the same filter, then puts the resulting list into a list as the only element. Now the arrow (meant to look like set inclusion!) says "for every element of the RHS list in turn, call that element [z]". [z] is the list containing the single element z, so the element "filter(==x)y" can only be called "[z]" if it contains exactly one element. Otherwise it gets discarded and is never used as a value of z. So the z's (which are returned on the left of the | in the list comprehension) are exactly the x's that make the filter return a list of length one.

That was my second version, my first version returns x instead of z - because they're the same anyway - and renames z to _ which is the Haskell symbol for "this value isn't going to be used so I'm not going to complicate my code by giving it a name".

could you explain your solution? I'm having trouble understanding :S
For those interested I've posted a haskell solution in a pointfree style: http://stackoverflow.com/questions/1344352/code-golf-duplicate-character-removal-in-string/2238192#2238192
Geoff Reedy
+1  A: 


165 characters from a pretty verbose language.

object c=getc(0),r={},b=r
while c>0 do
if find(c,r)then b&=c end if
end while
for i=1 to length(r)do
if find(r[i],b)=0 then puts(1,r[i])end if
end for
Matt Lewis
+1  A: 


A sed version with 30 characters, or 21 characters if you don't include the invocation of the command:

sed -r ':_;s/(.)(.*)\1/\2/;t_'

A java version using the same idea (should all be written in one line) which is 157 characters:

class T{public static void main(String[]a){String s=a[0];int l;
The sed doesn't work. It doesn't remove a character if it appears an odd number of times. Also, looping can be done with just `:; ...; t;`
The Java, by the same reasoning, doesn't work either.
+39  A: 
Frank Bollack
LOL, I love it.
hmmm - exactly how large is the resulting vi when saved? ;)
Ankur Goel
I guess. This is a lot like the "in my imaginary X language, it's only one character, because that's all the language does!" type answers. Sure, no "code"...because it's all in the design of the underlying system. Still, kudos for originality.
+1, because flowchart elements don't count as characters.
Stefan Kendall
The resulting VI is exactly 31.126 Bytes in size.
Frank Bollack
+2  A: 


118 characters actual code (plus 6 characters for the PHP block tag):

+1  A: 


98 char function, 115 char full program

f was written in the most readable format, and g aimed to reproduce f exactly but in a more terse manner.

Lowercase f and g functions are "verbose," and uppercase F and G are the "compressed" versions.

J is identical to G, but it is declared to show the necessary character count for a full program.


f = function(s)
    h=s:sub(1,1) -- head of string
    r=s:sub(2)   -- rest of string
    if r:find(h) then -- first character is repeated
        return f(s:gsub(h, '')) -- f(rest without any instance of h)
    elseif r > "" then -- there is something left in the rest of the string
        return h .. f(r) -- concatenate head with f(rest)
    else return h -- rest is empty string, so just return value left in head

F=function(s)h=s:sub(1,1)r=s:sub(2)if r:find(h)then return f(s:gsub(h,''))elseif r>""then return h ..f(r)else return h end end
--       1         2         3         4         5         6         7         8         9        10        11        12
-- 126 chars, compressed

g = function(s)
    return (r:find(h) and g(s:gsub(h, '')) or (r > "" and h .. g(r)) or h)

G=function(s)h=s:sub(1,1)r=s:sub(2)return r:find(h)and g(s:gsub(h,''))or(r>""and h ..g(r))or h end
--       1         2         3         4         5         6         7         8         9        10        11     
-- 98 chars, compressed

-- code-golf unit tests :)

J=function(s)h=s:sub(1,1)r=s:sub(2)return r:find(h)and g(s:gsub(h,''))or(r>""and h ..g(r))or h end print(J(arg[1]))
--       1         2         3         4         5         6         7         8         9        10        11     
-- 115 chars, full program


$ lua removedups.lua nbHHkRvrXbvkn
Mark Rushakoff
Lovely, with the ruler and everything. +1
Function F calls function f() instead of F(). Function G calls function g() instead of G(). Implementation has problems when string contains pattern magic characters `().%+-*?[]^$`. Try escaping matching string with `h:gsub('(%W)','%%%1')`.
How about this 96 characters LUA code:`s=arg[1];r='';for i in s:gmatch('.') do if s:match(i..'.*'..i)==nil then r=r..i end end;print(r)`
+6  A: 

another APL solution

As a dynamic function (18 charachters)


line assuming that input is in variable x (16 characters):

+2  A: 

Javascript 1.6


Shorter than the previously posted Javascript 1.8 solution (71 chars vs 85)

Thx @KennyTM for the improvement
+1  A: 

using C - 118 Characters

I think this answer is good as it will work for any string length.

main(int s,char *a[]){int i=0,j,c,d;while(s=c=a[1][i]){j=0;while(d=a[1][j])if(i!=j++)if(c==d)s=0;s&&putchar(c);i++;}}

and be removing the type defs of the variables, can get it down to 105 (new C winner i think :P) but my compiler is throwing errors with that!

What do you guys think?

I can reduce it to 111 using `i=0;main(s,A,j,c,d){char**a=A;while(s=c=a[1][i]){j=0;while(d=a[1][j])if(i!=j++)if(c==d)s=0;si++;}}`.
+1  A: 

D2 (templates): 195 197 199 + 17 characters

template F(alias s,int c,int i){static if(s>"")enum F=F!(s[1..$],c,i-(s[0]==c));else enum F=i?s:s~c;}template M(alias s,alias t=s){static if(s>"")enum M=F!(t,s[0],1)~M!(s[1..$],t);else enum M=s;}


template F(alias s,int c,int i){
    static if(s>"")
        enum F=F!(s[1..$],c,i-(s[0]==c));
        enum F=i?s:s~c;
template M(alias s,alias t=s){
    static if(s>"")
        enum M=F!(t,s[0],1)~M!(s[1..$],t);
        enum M=s;

+5  A: 

C: 83 89 93 99 101 characters

  • O(n2) time.
  • Limited to 999 characters.
  • Only works in 32-bit mode (due to not #include-ing <stdio.h> (costs 18 chars) making the return type of gets being interpreted as an int and chopping off half of the address bits).
  • Shows a friendly "warning: this program uses gets(), which is unsafe." on Macs.


main(){char s[999],*c=gets(s);for(;*c;c++)strchr(s,*c)-strrchr(s,*c)||putchar(*c);}

(and this similar 82-chars version takes input via the command line:



++; this is why I love C.
David X
+2  A: 

C# (53 Characters)

Where s is your input string:

new string(s.Where(c=>s.Count(h=>h==c)<2).ToArray());

Or 59 with re-assignment:

var a=new string(s.Where(c=>s.Count(h=>h==c)<2).ToArray());
+11  A: 

J (16 12 characters)



(~.{~[:I.1=#/.~) 'nbHHkRvrXbvkn'

It only needs the parenthesis to be executed tacitly. If put in a verb, the actual code itself would be 14 characters.

There certainly are smarter ways to do this.

EDIT: The smarter way in question:

(~.#~1=#/.~) 'nbHHkRvrXbvkn'

12 characters, only 10 if set in a verb. I still hate the fact that it's going through the list twice, once to count (#/.) and another to return uniques (nub or ~.), but even nubcount, a standard verb in the 'misc' library does it twice.

Another way to do it in 12 characters is `(][email protected]~:#])`, with the advantage of only going through the list once.It just copies from the list the `not` of the nubsieve, then removes those elements from the list.
@David: And more emoticons, too.
Dave Jarvis
+2  A: 

Haskell Pointfree

import Data.List
import Control.Monad
import Control.Arrow

The whole program is 97 characters, but the real meat is just 23 characters. The rest is just imports and bringing the function into the IO monad. In ghci with the modules loaded it's just

(liftM2(\\)nub$ap(\\)nub) "nbHHkRvrXbvkn"

In even more ridiculous pointfree style (pointless style?):

main=interact$liftM2 ap liftM2 ap(\\)nub

It's a bit longer though at 26 chars for the function itself.

Geoff Reedy

Lua, 97 Characters

i=...repeat c=i:match("(.).-%1")i=c and i:gsub(c:gsub("(%W)","%%%1"),"")or i until not c print(i)

The D programming language version 2, 68 characters:

auto f(S)(S s){return array(filter!((a){return s.count(a)<2;})(s));}
+2  A: 

Shell/Coreutils, 37 Characters

fold -w1|sort|uniq -u|paste -s -d ''
+2  A: 


Tested with WinXP DOS box (cmd.exe):

    xchg cx,bp
    mov al,2
    rep stosb
    inc cl
l0: ; to save a byte, I've encoded the instruction to exit the program into the
    ; low byte of the offset in the following instruction:
    lea si,[di+01c3h] 
    push si
l1: mov dx,bp
    mov ah,6
    int 21h
    jz l2
    mov bl,al
    shr byte ptr [di+bx],cl
    jz l1
    inc si
    mov [si],bx
    jmp l1
l2: pop si
l3: inc si
    mov bl,[si]
    cmp bl,bh
    je l0+2
    cmp [di+bx],cl
    jne l3
    mov dl,bl
    mov ah,2
    int 21h
    jmp l3

Assembles to 53 bytes. Reads standard input and writes results to standard output, eg:

 programname < input > output
+4  A: 

Golfscript(sym) - 15

||    |    |    |    |    |    |    |    |    |    |    |    |    |    |  |
|0         10        20        30        40        50        60        70 |
|                                                                         |
+1 for the ruler
I pinched it from the ruby answer :)
+2  A: 

sed, 41 chars


Usage: $ echo nbHHkRbvnrXbvkn | sed -r ':;s/((.).*\2.*)\2/\1/;t;s/(.)(.*)\1/\2/;t'

With contributions by gnarfpyon ;).

Congrats from t0mm13b ;) +1 from me...
+1  A: 

LilyPond, 78 chars

z=#(ly:gulp-file"A")#(print(string-delete z(lambda(x)(>(string-count z x)1))))

Usage: $ cp input.in A; lilypond this.ly


F#, Non-Code-Golf version using lists:

16 ilnes using lists:

let rec removefromlist v l = 
    match l with 
    | h::t -> 
        let (res, removed) = removefromlist v t
        if h = v then (res, true)
        else (h::res, removed)
    | [] -> ([], false)
let rec removedups unique tail = 
    match tail with
    | h::t -> 
        let (u_res, u_removed) = removefromlist h unique
        let (t_res, t_removed) = removefromlist h t
        if (t_removed || u_removed) then removedups u_res t_res
        else h::removedups u_res t_res
    | [] -> []
removedups [] (Array.toList("nbHHkRvrXbvkn".ToCharArray()));;

F# - 185 characters - More terse version without lists:

let s = "nbHHkRvrXbvkn"
let n c (s:string)=
    let mutable i = 0
    for x in 0..s.Length-1 do
        if s.[x]=c then i<-i+1
String.collect (fun c -> if n c s>1 then "" else c.ToString()) s
Igor Zevaka

GW-BASIC - 117 chars

1INPUT I$:FOR I=1 TO LEN(I$):S$=MID$(I$,I,1):C=0
2P=INSTR(P+1,I$,S$):C=C-(P>0):IF P GOTO 2
3IF C=1 THEN ?S$;
Carlos Gutiérrez

OCaml, 93 characters (wc -c).

open String
let f s=let r=ref""in iter(fun c->if not(contains!r c)then r:=!r^(make 1 c))s;!r

# Python, 67 characters, in a giant generator.
f=lambda s:''.join([c for c in s if len([d for d in s if d==c])<2])

string = "nbHHkRvrXbvkn"
print f(string) => "RrX"

F#, 94 chars

let f(a:string)=new string(Seq.toArray(Seq.filter(fun(c:char)->a.IndexOf c=a.LastIndexOf c)a))
printfn "%s" (f "nbHHkRvrXbvkn")

Python, 50 chars (based on SHiNKiROU 67 char version)

f=lambda s:''.join([c for c in s if s.count(c)<2])
Andrea Ambu