A dupe is a message with the same message-id that arrives for the
This can happen due to multiple reasons:
- the spam sender has your mail address multiple times on his list
and sends out spam with the same message-id
- you get messages from a mailing list with multiple accounts
- you changed your Outlook account settings and Outlooks thinks
that it needs do download all messages again
The Dupe Checker has an expiration age for cleaning old message-ids.