Why is my keyboard typing double letters?

When one press types two letters, the key's switch is usually sending two signals for one press. That is called key chatter. The usual causes are dirt or crumbs under the key and a worn or damaged switch; less often it is a setting, a weak wireless connection or the keyboard's own software. Test the key first, then work through the fixes below from the free ones to the costly ones.

A worn or dirty switch can break contact for a moment during one press. The keyboard reports two presses a few milliseconds apart, and you see a double letter.

1. Test the key

Open the keyboard double-press test and press the suspect key slowly, about twenty times, one press at a time. It lists the time between presses of the same key and flags gaps shorter than anyone presses on purpose.

Short gaps on one or two keys: those keys chatter. Start at step 3.
Short gaps on many keys, or doubles that come and go: start at step 2.
No short gaps, but letters still double while you type: a setting is the likely cause. Start at step 2.

The test sees key events, not the switch's electrical signal, so one fast double on its own does not prove a broken switch. A pattern that repeats on the same key does.

2. Rule out the computer and the settings

Another port or computer. Plug the keyboard, or its wireless receiver, straight into another USB port, not a hub. If you can, try a second computer. If the doubles stop there, the keyboard is fine.
Wireless keyboards. Charge it or fit fresh batteries, and move the receiver closer, with nothing in the way.
Repeat delay. A held key starts repeating after a short delay; if that delay is very short, a slightly long press can type twice. On Windows, open Keyboard in Control Panel and move Repeat delay toward Long. On a Mac, open Apple menu > System Settings > Keyboard and drag Delay until repeat toward Long.
Filter Keys on Windows. Filter Keys makes the keyboard ignore brief or repeated keystrokes. On Windows 11 it is in Settings > Accessibility > Keyboard. It can hide chatter while you wait for a repair, but it also slows normal typing.

3. Clean the key

Unplug the keyboard or switch it off. Turn it upside down and tap it gently, then blow compressed air under the key from a few angles. If its keycaps come off, lift the cap with a keycap puller and clean around the switch. Never pour liquid into a keyboard. Test the key again afterwards.

4. Update the keyboard's software

If your keyboard has its own app, install the latest firmware. Some gaming keyboards also offer a debounce setting: the time the keyboard waits before it accepts a second press of the same key. A slightly longer debounce filters chatter.

5. Replace the switch or the keyboard

Hot-swap mechanical keyboards: pull the chattering switch and fit a new one of the same type.
Under warranty: contact the maker before you open the keyboard. Note which keys double and how often, from the test.
Out of warranty: a soldered switch can be replaced by someone with the tools, or it is time for a new keyboard.

On a laptop or MacBook

One key rarely comes out on its own. Most laptop keyboards are a single part, often fixed into the case, so a lasting repair replaces the whole keyboard. Clean as in step 3 but leave the keycaps on: laptop caps clip onto thin plastic hinges that break easily.
Windows laptops: open Device Manager, expand Keyboards, right-click the keyboard, choose Uninstall device, then restart. Windows installs the driver again as it starts, which clears doubling that came from the driver rather than the key.
MacBooks from 2015 to 2019 with the thin "butterfly" keyboard (the 12-inch MacBook, and the MacBook Air and MacBook Pro of those years): letters that repeat, keys that stick and keys that do not respond were common enough that Apple repaired eligible keyboards free from June 2018, and from May 2019 covered every butterfly keyboard for four years after the laptop was sold. Apple's page for that programme now says it has ended, so a repair today is paid unless other cover applies. Apple moved to a scissor-switch keyboard from late 2019 (16-inch MacBook Pro) and 2020 (the others).

A mouse button doing the same? Read why a mouse double-clicks. To check every key at once, use the keyboard test.