Right now, when entering or adding a new term, you can get a message 'Term already exists. Merge? Duplicate?'. However, you cannot always know beforehand exactly which is the other term. So when you decide to merge, for example, you find that it was not the right thing to do. But it is too late, you have to do manual steps to restore the situation.
You should have all the details of the 'to-be-merged-with-term' before deciding on which option (merge or duplicate).