svn cp WC URL fails with duplicate file

Hej!

I'm trying to tag the current state of my working copy and it always
fails on the same file:

wc $ svn cp . https://server/repo/tags/test_1 -m "tag clean copy of WC
to URL"
svn: Übertragen schlug fehl (Details folgen):
svn: Datei
»/repo/tags/test_1/Unitech_PXA320/SRC/HUW/IT5x00_ce6/mt9v022_imager.cpp«
existiert bereits

(svn: file »..../mt9v022_imager.cpp« already exists) There is such a
file in the WC but https://server/repo/tags/test_1 doesn't exist before
the copy.

This is subversion client 1.5.5, subversion server 1.2.3 (r15833)

The working copy is clean, "svn status -q" doesn't show any modified
files, wc/Unitech_PXA320/SRC/HUW/ is uptodate (can't do a complete
update). The WC consists of 64510 versioncontrolled files out of a
total of 235897 files (only partly ignored).

I've found http://svn.haxx.se/users/archive-2008-11/0833.shtml which
seems to imply that "svn cp WC URL" isn't possible if the WC isn't
completely uptodate. But wouldn't this render tags mostly useless? We
are all working in trunk, updating frequently and tagging whenever a
release is ready, but testing takes some time and new commits during the
testing shouldn't make it to the tag. I don't find this issue in the
FAQ or bugtracker so perhaps I'm misinterpreting the above thread?

Any pointers how "svn cp WC URL" is supposed to be used are welcome!

Bye,
  Vitus

-- 
Vitus Jensen, Germany, Earth, Universe (current)