I reported bug 7663 some time ago:

  #7663 23.2; duplicate entries in vc-dir-mode (Windows) due to
  backslash from svn output

Here is a patch with a one line fix.

The subversion command svn.exe I use on windows output backslashes. In
all other places in vc-svn.el a call to expand-file-name causes these
to replaced with forward slashes.

The problem was that same file ends up appearing twice in the vc-dir
buffer.

2011-02-24  Vagn Johansen  <vagn.johansen@gmail.com>

	* vc/vc-svn.el (vc-svn-after-dir-status): Replace backslash
	with forward slash in output from svn commmand.