Does this seem like a possible answer to your situation?
(from http://www.uwplatt.edu/csse/tools/subversion/subversion-help.html)
  "Failed to add file '(name here)': object of the same name already
  exists.
  
  or
  
  "Can't open file 'folder.svn\tmp\text-base\file.svn-base':
  The system cannot find the file
  specified."
  
  Both mean that two files in the same
  folder have the same name except for
  capitalization; for example
  "Readme.txt" and "README.TXT". Unix
  and Subversion are case-sensitive, so
  the files are considered to be
  completely unrelated. But in Windows
  is not case-sensitive, so when it
  tries to update README.TXT on top of
  Readme.txt (say), it breaks.
  
  The surest way to fix the problem is
  to log in to a Unix system (such as
  io.uwplatt.edu) and use the unix notes
  to check out the repository there. You
  can then use the svn mv command to
  rename one of the files. If you are in
  the middle of trying to add a file to
  your repository, you might try using
  TortoiseSVN->Rename... to rename the
  existing file to something entirely
  different and then updating. Note that
  you need to use the TortoiseSVN rename
  commands; merely renaming the file in
  Windows Explorer won't fix your
  problems.
I had this same error, and using Tortoise SVN's repo browser was able to confirm that there were two files in the repository with the same name only varying by upper/lowercase letters.  You can also fix this issue through the repo browser by deleting one of the files (you can do a diff first through repo browser to make sure you have what you need).