Looks like as of OSX 11, GCController implements supportsHIDDevice: which should allow for filtering duplicates. This is used within SDL2 but likely a bug or order of operations issue depending on whether GC or IOKit detects first.

Wow, 6 years ago and find a post about a current issue. We are hitting the same issue with an SDL2 based app whereby MFi / IOKit compatible controllers (PS/XB/Switch Pro) get detected twice resulting in the appearance that 2 gamepads are connected when there is only 1 in actuality. Is there a method for filtering this out to avoid duplicates?