The highest user_id should be the same in each of these tables. If any of them are higher than the xf_user table then that explains the error. A quick fix is to increase the AUTO_INCREMENT value in xf_user by selecting that table and clicking the Operations tab.
I only had two users on this install, myself and a dummy account. The dummy account didn't import properly, apparently, after the upgrade from 1.0.4 to B1 but I didn't check it till the upgrade to B2. I did manually delete it from the database because it wouldn't let me from the ACP (saying the user didn't exist) and tried to re-create it via the ACP and got this error. I ended up just re-registering the account from the forum interface.