Not very good things (if Kovid did not trap it already)
The AutoAdd folder should NEVER intersect with a 'Known Library" (any of those on the switch list). Of coarse, there is no clean way to trap any 'unknown Library' (not on the list).
If you need to
Merge, select the books in Calibre and use the context menu:
Copy To Library (<optional: delete <= 'Move'>).