duplicate tag created when synchronising with upstream CVS repository

[sharkcz]

transferred from Fedora's bugzilla, for all details (logs, config, test repo, ...) please see  https://bugzilla.redhat.com/show_bug.cgi?id=498786

I used tailor to create a darcs repository based on a CVS repository. I committed some changes to my darcs repo, and then used tailor again to pull in any upstream changes that had been committed since the previous tailor run (as it turned out, there were none). Tailor then re-tagged the repository, even though it had already tagged it, with the same tag, the first time.