Can't reproduce. Extra slashes immediately after the domain are never passed to mod_rewrite even with DirectorySlashes off -- I haven't checked whether it's Opera or Apache that's removing the slash). But otherwise, everything works fine:
RewriteBase /
RewriteCond %{REQUEST_URI} ^//+(.*)$ [OR]
RewriteCond %{REQUEST_URI} ^(.*/)/+$
RewriteRule .* http://domain/%1 [R=301,L]
Request for http://localhost//abc/b//
127.0.0.1 - - [17/May/2010:12:26:36 +0100] [localhost/sid#c08fb0][rid#2756540/initial] (3) [perdir C:/HTTP/htdocs/] add path info postfix: C:/HTTP/htdocs/abc -> C:/HTTP/htdocs/abc/b/
127.0.0.1 - - [17/May/2010:12:26:36 +0100] [localhost/sid#c08fb0][rid#2756540/initial] (3) [perdir C:/HTTP/htdocs/] strip per-dir prefix: C:/HTTP/htdocs/abc/b/ -> abc/b/
127.0.0.1 - - [17/May/2010:12:26:36 +0100] [localhost/sid#c08fb0][rid#2756540/initial] (3) [perdir C:/HTTP/htdocs/] applying pattern '.*' to uri 'abc/b/'
127.0.0.1 - - [17/May/2010:12:26:36 +0100] [localhost/sid#c08fb0][rid#2756540/initial] (4) [perdir C:/HTTP/htdocs/] RewriteCond: input='/abc//b//' pattern='^//+(.*)$' => not-matched
127.0.0.1 - - [17/May/2010:12:26:36 +0100] [localhost/sid#c08fb0][rid#2756540/initial] (4) [perdir C:/HTTP/htdocs/] RewriteCond: input='/abc//b//' pattern='^(.*/)/+$' => matched
127.0.0.1 - - [17/May/2010:12:26:36 +0100] [localhost/sid#c08fb0][rid#2756540/initial] (2) [perdir C:/HTTP/htdocs/] rewrite 'abc/b/' -> 'http://domain//abc//b/'
127.0.0.1 - - [17/May/2010:12:26:36 +0100] [localhost/sid#c08fb0][rid#2756540/initial] (2) [perdir C:/HTTP/htdocs/] explicitly forcing redirect with http://domain//abc//b/
127.0.0.1 - - [17/May/2010:12:26:36 +0100] [localhost/sid#c08fb0][rid#2756540/initial] (1) [perdir C:/HTTP/htdocs/] escaping http://domain//abc//b/ for redirect
127.0.0.1 - - [17/May/2010:12:26:36 +0100] [localhost/sid#c08fb0][rid#2756540/initial] (1) [perdir C:/HTTP/htdocs/] redirect to http://domain//abc//b/ [REDIRECT/301]
Note: consider not hard-coding the host:
RewriteCond %{HTTP_HOST} !=""
RewriteCond %{REQUEST_URI} ^//+(.*)$ [OR]
RewriteCond %{REQUEST_URI} ^(.*/)/+$
RewriteRule .* http://%{HTTP_HOST}/%1 [R=301,L]
Also note that the inner "//" was not replaced. You will to add another rule to replace inner slashes.
NEW EDIT:
OK, this seems to work for preventing URLs starting or ending with //:
RewriteEngine on
RewriteBase /
RewriteCond %{HTTP_HOST} !=""
RewriteCond %{THE_REQUEST} ^[A-Z]+\s//+(.*)\sHTTP/[0-9.]+$ [OR]
RewriteCond %{THE_REQUEST} ^[A-Z]+\s(.*/)/+\sHTTP/[0-9.]+$
RewriteRule .* http://%{HTTP_HOST}/%1 [R=301,L]