An update is being deployed to enforce uniqueness of mailnickname assigned to an Office 365 Groups and is scheduled to be completed by the end of August.

Prior to this update, this was not enforced and then caused potential collisions when you were using AAD Connect groups writeback.

Any Office 365 Groups created before this update will not be affected by the deployment.

There is no change in the way you create and manage Office 365 Groups.