Key chatter
Also called double typing · switch chatter
Key chatter is one key press registering as two or more characters. It usually means a mechanical switch is dirty or worn and its contacts bounce when they close.
Chatter is different from key repeat, which is the operating system repeating a key you are still holding. Cleaning the switch fixes some cases and replacing it fixes the rest.
In practice
Chatter shows up as doubled letters while typing: tthe, helllo, a space bar that adds two spaces. It usually starts on the keys you use most and gets worse over time. Mechanical switches are most prone to it because their metal contacts wear and collect dust. Membrane keyboards can chatter too when the rubber dome or the contact sheet beneath it degrades.
How to check
Press the suspect key slowly and deliberately in the keyboard test while watching its press count. One press should add one. If the count jumps by two from a single tap, the switch is chattering. Compare with a key you rarely use. Cleaning under the keycap sometimes helps. A replacement switch is the lasting fix on hot swap boards.