In MMEX Management Tree one can see duplicate objects, like servers or mailbox databases and the following is logged in the EMWMigration.log file when doing a refresh:
Info aeEMWGUIShared.SQLStoreStruct.AddMailboxDBCopy|Adding mailbox database copy: CN=[DB_Name],CN=[DB_Name],CN=Databases,CN=Exchange Administrative Group (FYDIBOHF23SPDLT),CN=Administrative Groups,CN=[Exchange_org],CN=Microsoft Exchange,CN=Services,CN=Configuration,DC=[domain_name],DC=com
Error aeEMWGUIShared.SQLStoreStruct.AddMailboxDBCopy|-2147217900-[Microsoft][ODBC SQL Server Driver][SQL Server]Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression., (CN=[DB_Name],CN=[DB_Name],CN=Databases,CN=Exchange Administrative Group (FYDIBOHF23SPDLT),CN=Administrative Groups,CN=[Exchange_org],CN=Microsoft Exchange,CN=Services,CN=Configuration,DC=[domain_name],DC=com)