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Thank you @peregrine. The first link (http://vanillaforums.org/discussion/comment/220447/#Comment_220447) helped. I was getting that same error, and running the sql command
alter table GDN_Tag drop index UX_Tag appears to have taken care of the problem.
Okay, so this
Ahhh well, tried it on a different install on a different dev machine and the problem disappeared, so never mind for now.