Edited Netaddress Email Keeps Duplicating

The behavior between the Exchange and Netaddress accounts indicates an IMAP server or synchronization interaction, not an Outlook feature. Editing a received message changes the stored item; with IMAP, Outlook must synchronize that replacement to the provider. If Netaddress does not handle that operation consistently, each synchronization can create another copy. Stop editing messages in that mailbox until testing is complete. Update Outlook, create a new Outlook profile containing only the Netaddress account, and reproduce with one harmless test message. Compare the same edit in Netaddress webmail. If duplication occurs there, the provider owns the behavior; if it occurs only in Outlook, collect a synchronization log and contact Microsoft support. Office and Outlook 2016 reached end of support in October 2025, so Microsoft no longer supplies bug or security fixes. The safest long-term step is moving to a supported Outlook version. Use categories or flags instead of editing received subjects.