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I've skimmed your issue, and indeed moving "emit_signal("skillplayed")" lower down seems like it would have caused this issue. But I tested reversing your fix and this autoattack bug still happened, so I think coroutine race conditions are definitely at play. I think that the reason I gave for why my fix works was incomplete. My fix works because it limits possible outcomes by using a variable which is not likely to be subject to a race condition.(accidental success)
I am uncertain, but I think that my fix will inadvertently undo your fix, so it can only be applied safely to non-linux games. As both of us have pointed out a proper fix involves a significant rewrite of combat.