This is by design. Our cmdlet mirrors the native PowerShell cmdlet (cmdlet: 'Add-adgroupmember'), which accepts readding of objects to the group's membership even thought they are already members.
An enhancement request (TF00727667) has been created detailing the desired feature:
“ActiveRoles PowerShell should display error message and not allow group members to be added if they are already a member.”
The product team will evaluate the request and this feature may become available on a future release of the product.
Please refer to this article for updates or contact support referencing the Enhancement Request ID: TF00727667.