First things first: I only have one single file that exposes this weird behaviour, so this does not affect everyone.
When I select one single player in setup mode and do “Right click > Duplicate player”, the new player gets added to each and every existing layout, creating quite a mess.
Has anybody ever heard of this?
I will send the file to @dspreadbury, as it reproducibly exposes this in this file. I immediately tested it on another file, but everything is ok there.
Edit: This is a native Dorico file, no XML-import.