A client has a distribution group in Exchange Online with many members that restricts who can send to the group. This is a common scenario and works quite well.
Configuring specified senders is done in Delivery management. In the example below, Byron Wright and Jeff Smith are allowed to send. Normally to add another sender, you search for them, add, and then save.
When we were adding an additional user, we got the following error:
the identity "<user>". Please specify a unique value.
This was verify confusing because we knew all of the email addresses were unique. However, when searching, I found a document mentioning the same error for creating rules in OWA. The cause of the error was multiple recipients having the same display name.
Sure enough, we went through the list of specified senders and found several that had duplicates with the same display names (student and staff accounts for example). After we changed the duplicate display names and synced them up to Azure AD we could add and remove specified senders as expected.
The recipient indicated in the error message is not necessarily the identity with the duplicate Display Name. If you attempt to remove users one by one and save the changes, the successful save indicates which is a duplicate. If you're using the method, make sure to note the allowed senders before you start editing.
Otherwise, you can also use the Admin center search bar to see if there are duplicates for a name. This lets you search across recipient types for users, contacts, groups, and teams.
There seems to be some inconsistency in how the uniqueness of users is evaluated. It seems that when you initially add a user it's based on the email address and the display name issue only pops up when you go to edit the list afterwards. For example, we could add Bob@adatum.com (Bob Smith) as a sender even though there was another Bob Smith. But after doing that we'd get the error if we attempted to edit the list.
