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 ti...
Updated Jul 01, 2019
Version 2.0
Blog Post
Duplicates may occur in Exchange when a message is sent to a user and to a DL containing that user. This usually happens if the DL needs to be expanded on a different server or if it is a hidden DL.
Also there is no way to turn off duplicate detection.
Also there is no way to turn off duplicate detection.