Those two emails are copied by "DelegateSentItemsStyle" and "MessageCopyForSentAsEnabled/MessageCopyForSendOnBehalfEnabled"
I check in my lab, the "DelegateSentItemsStyle" key doesn't exist by default.
So, if you enable this key on your client, and don't want to disable this one. You will need to disable the MessageCopyForSentAsEnabled and MessageCopyForSendOnBehalfEnabled on this mailbox. In this way, there will not exist duplicate emails.
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