How to use the double typing test
- Press keys slowly, one at a time, about one a second. If you already know which key is the problem, press only that key twenty or thirty times.
- Watch the line of typed characters: a doubled press is highlighted, and the key is outlined on the drawing.
- After 30 presses the test gives a verdict.
For every press, the test measures the gap since that same key was last released. A gap shorter than the limit, 30 ms unless you change it, is counted as a double press. When you press slowly, a finger never comes close to that: lifting off a key and pressing it again takes far longer.
Why a keyboard types double letters
Inside a key switch, two metal contacts meet when you press and part when you let go. They do not meet cleanly: they bounce for a moment, and the keyboard’s firmware waits for the signal to settle before it reports anything. That wait is called debouncing. QMK, a popular open-source keyboard firmware, waits 5 ms by default.
When a switch wears out, gets dirty or has something spilled into it, the bouncing can outlast that wait, and one press comes out as two. This is called key chatter. It usually affects one or two keys, often the ones you use most.
How to fix a key that types twice
- Clean it. Take off the keycap and blow out dust and crumbs with compressed air. Some people report that pressing the key quickly a few dozen times afterwards also helps.
- Swap the switch. On a keyboard with hot-swap sockets, swap the chattering switch with one you rarely use, or replace it. If the fault moves with the switch, the switch is the cause.
- Raise the debounce time in the keyboard’s software or firmware, if it has that setting. A few milliseconds more can hide mild chatter.
- Use Filter Keys on Windows. Under Settings → Accessibility → Keyboard, Filter Keys can ignore brief or repeated keystrokes. It works for every keyboard on the computer and hides the symptom rather than curing it. The nearest setting on a Mac is Slow Keys, under System Settings → Accessibility → Keyboard, which adds a delay before a press is accepted.
- Check the warranty. Chatter on a new keyboard is a fault worth reporting to the seller or maker.
Test again after any fix. With Filter Keys on, this test stops seeing the double presses too, because Windows removes them before any program receives them.
Double typing or key repeat?
When you hold a key down, the computer repeats it after a short delay. That is normal, and this test ignores it. If letters repeat when you hold a key only briefly, lengthen the repeat delay in your keyboard settings. Double typing is different: two presses from one quick tap, with no hold at all.
If this test shows no double presses but letters still double in one particular program, the cause is most likely in software, such as that program’s settings, a macro tool or a remote desktop connection.
To check every key on the keyboard at once, use the keyboard test. For a mouse that clicks twice, use the mouse double click test.