Should both tags remain, or should we make them synonyms?
Which should be the primary tag, if we keep one of them?
Do we need either of the tags?
Karl's proposal is what I would have said too. As for the last part:
Yes we need the tags, simply because somebody entered them. It makes sense to distill them into a single tag (possibly with many synonyms) but if anybody finds any given tag useful, I don't want to remove it. Besides, it would be futile - without any synonyms to route it elsewhere, the next user can create the tag(s) again.