The administration and maintenance of Microsoft Exchange Server to ensure secure, reliable, and efficient email and collaboration services across an organization.
Ok, so you sent the same message to both your work address and your gmail account from an internal account?
In that case, I would expect to only get one since duplicate detection would kick in
Old article but still applies as far as the checks:
https://techcommunity.microsoft.com/t5/exchange-team-blog/how-does-duplicate-detection-work/ba-p/610180