Don't have Telegram yet? Try it now!
https://nishantrana.me/2016/11/10/crm-internal-exception-singleton-retrieve-query-should-not-return-more-than-1-record-query-system-data-sqlclient-sqlcommand-while-importing-solution-in-dynamics-365-crm-2016-or-earlier/
Crm Internal Exception: Singleton Retrieve Query should not return more than 1 record. Query: System.Data.SqlClient.SqlCommand while Importing Solution in Dynamics 365 \CRM 2016 or earlier