Duplicate detection of delivered messages is done by the Exchange store. The store does duplicate detection based on two properties on the message - the Internet Message Id and the client submit ...
Updated Aug 26, 2026
Version 3.0
Blog Post
In your example "a SMTP-server on the way to me duplicates a message" each message would have an altogether different message ID once it leaves that smtp relay, which would allow delivery into your exchange environment as it should be seen as a unique mail message