I was looking through the list of duplicate users labelled like Jon Bannon_1, Jon Bannon_2, etc. and realised that a few users are obvious duplicates (e.g. Jon Bannon and MKO and Daniel Gottesman) whom I merged. Sometimes this strategy failed though (I realised on time), since once LifeWorks claimed to be British and the other wrote his About Me in Chinese!
Some users were duplicates which could be seen from their identical email hashes, e.g. Matthew.
In the remaining users, we were very lucky because for almost all of them, only one of the duplicate usernames had any posts at all! Except for three users, Paul_1, Paul_2, and Paul_3, I'll figure out what to do with them later.
Now, since for each set of duplicates, besides the Paul set, e.g. Thomas_1, Thomas_2, since only one, e.g. Thomas_2, had any posts at all, I think it would be safe to rename the one user per set, e.g. Thomas_2 to Thomas.
So, can we have a "Rename User" plugin which can be used by Super-Administrators? This is assuming that making this plugin would take less time than renaming the users, i.e. less than about 2-3 hours.