Here is the question: How do I remove any trace of a SQL server from a 2013 farm?
Why I ask: I have a 2013 farm. I accidentally created a new sit with the content db sitting on a sql server that is a member of a 2016 farm instead of the original SQL server on the 2013 farm. I am testing upgrades from SharePoint 2013 to 1016 and of course I am getting the error "another object in this farm already contains this ID..." since the SQL server in the 2016 farm thinks it is also in the 2013 farm now.
What I have tried: I have used the CA GUI to remove the SQL server from the farm, but I still get the ID conflict error when I try to mount the copied DB.
Thanks for any advice.