Sometimes, users create a second account, perhaps because they don't realise they can change the email address associated with their original account. And then, once they discover this, they don't want to abandon the history associated with either account.
The best way forward in this case would be to merge one of their accounts into the other one, so it looked like the target account had done all the actions of both. In Bugzilla's contrib directory, there is a script called merge-users.pl. While code in this directory is not officially supported by the Bugzilla team, this script may be useful to you in solving the above problem.
This documentation undoubtedly has bugs; if you find some, please file them here.