At work I'm on a really old version of Gnome and had the same issue. I noticed I could type consecutive characters if I added a pause of 1 to 2 seconds in between each press.

Of course this is not ideal. I finally found that some how the Accessibility option called Bouncing Keys (or Bouncy Keys?) was turned on which caused this behavior. Turning it off made things work as usual.

Not sure if this is your problem as well, but it's worth a look

I would chuck your accessibility options on the login screen, and maybe any input method that is between your keystrokes and the input target. I would have some more specifics for older gnome but these days I just use defaults all the time 😅. Do you havr more than one language installed in gnome settings? Or a language with special input modes?

Does the keyboard work as expected if you switch to a text console (eg. Ctrl+Alt+F2)?

Are you on the default stable branch?

If yes and yes, you can try reinstalling the libinput package, and make sure you don't have any libinput package installed from AUR.

Another thing you can try is to check your Gnome third party extensions if you have any, there were some issues I saw mentioned because some extensions haven't been updated to work with 45.

Not going to help a lot but may point to a motherboard issue - I've experienced this under Windows and a reboot has solved it.