While moving messages from previous ISP to Proton using Mail Bridge (from multiple accounts into separate folders over a few days), the process('es) had to be restarted multiple times, and resulted in many (1000s) duplicates. If not for the Mail Bridge and my previous MUA's facilities to deal with dups, it would've been painful and time consuming to track them through Proton Mail UI.

There currently seems to be no facility to deal with duplicate messages.