Hi @Srbh_TheComputerGuy ,
1.According to my test, I create a new Dynamic distributoin group in Office 365, and set it display name and email address to be the same as a dynamic distriobution group in on-premises Exchange server. When I send an email to this email address, both dynamic distribution group received email. If you do not delete the dynamic distribution group with the same email address in the on-premises Exchange server. Then the mail sent to the dynamic distribution group in Office 365 may be accidentally sent to other recipients.
2.According to me research and test, I found a script could export all distribution group and all members in Exchange and Offcie 365. You could judge whether the distribution group is nested dynamic distribution group according to the exported members.
For more information: Export all distribution Group and all members in Exchange and Office 365
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