Key repeat
Also called typematic rate · repeat delay
Key repeat is the operating system typing a character over and over while you hold its key down. The delay before repeating starts and the speed of repeating are system settings, not properties of the keyboard.
If characters repeat when you tap a key briefly check the repeat delay setting before you blame the hardware.
In practice
Key repeat is what lets you hold down a key to type a row of dashes. The operating system waits for a short delay, then repeats the character at a set rate until you let go. A delay set very short makes a normal press produce two characters, which looks like a faulty key but is only a setting.
How to check
In Windows, find the repeat delay and rate in the Control Panel keyboard settings. On a Mac they are under Keyboard in System Settings. Set the delay one step longer and type normally for a while. If the doubled letters stop, it was the setting. If one key still doubles while others don’t, that key is chattering instead.