I have a question more than an issue, I have a scenario where I perform a database attach content DB migration and this works out OK, then the site goes into a period of UAT and after a week or so the customer would like to do this again for the production server. I then take another backup of the content database and attach it to the 2013 farm and upgrade it again (after discarding the original tested and upgraded DB).
So then I upgrade again with Mount-SPContentDatabase and it goes off and upgrades again, this seems to all work out in the test environment but perhaps I'm missing something here such as references in the SharePoint_Config DB?