I'm experiencing this too. Only fixed by restarting Xcode. Have to do it a handful of times an hour. For me restarting Xcode didn't seem to fix it. Maybe the trigger happened really quickly for me. But Xcode 12 beta 4 seems to be fixed for me! I hope so.

Oh my god, finally somebody else is seeing this! It seems like all keypresses are being duplicated. Often the second one results in the alert beep, but things like CMD-T will open *two* tabs and CMD-W will close *two* tabs. CMD-N to add a new file shows the template chooser but also an alert saying "An assistant session is already running on this window". What on earth is going on? Like the original poster, I only see this on Xcode Version 12.0 beta 3 (12A8169g). But I am seeing it on macOS 10.15.6 (19G73).

I have exactly this issue. Interface Builder has "forced" my split view controller into double width style. When run on iOS 13 it all seems to work, when run on iOS 14 I get problems because it isn't behaving like it used to. In my app (Adaptivity) I deliberately want to be able to show the user how the Classic style split view controller behaves because it is different. For example, in a double/triple column split view the primary view controller is actually wider than what appears on screen (420 points instead of 320) with a leading margin which is 100 points larger than the trailing margin to cancel out the effect. It would be really useful if we could select 'Classic' in Interface Builder to keep using the old initialiser.