I have seen this error multiple times in previous versions, both in my own games and here in the various forum bug threads, but I managed to reproduce it consistently. It has to do with relabelling copies of classes as a different class type.
Error Number: #235
Error Message: Cannot add or insert the item Covenant (P) in more than one place. You must first remove it from its current location or clone it. parameter node: none.
Affected Window: Class design
Steps to reproduce: In the attached DB, choose the Jump Destroyer "Covenant" class. Choose copy class. In the drop down menu in the top, reassign the copy as a destroyer escort.
Other info:
Version: 1.9.3, started in 1.9.0, no mods
Decimal separator: dot
Conventional start, Real stars, 31yr campaign