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