Open a service request so that the Starling Product Team can assist with identifying if the ARS instance is already joined.
You can work with that person to have them unjoin.
If required the Starling Product Team can unjoin the instance if it's determined that is the only option so that ARS can be rejoined.
ARS Change Requests 125202 and 127534 have been raised to change the behavior so that a GUID it added to the instance name that will make each join unique. At this time there is no ETA on when that fix will be included in ARS. (August 30, 2019)