One press, two letters is almost always switch chatter — worn or dirty contacts bouncing inside the switch faster than the firmware ignores them. It’s the most common fault on aging mechanical keyboards, it’s diagnosable in under a minute, and it’s usually fixable for free. Here’s how to confirm it and then work the fix ladder.

What’s physically happening

Inside every keyswitch, two metal contacts meet when you press — and they don’t meet cleanly. Like a dropped ball, they bounce: a microscopic open-close-open-close chatter lasting 1–20 milliseconds. Keyboard firmware knows this and applies a debounce window — ignore all state changes for N ms after the first contact. Double typing happens when the bounce outlives the window:

• Switch wear — the contact springs lose tension with use, so the bounce gets longer and sloppier. The keys you press most degrade first, which is why ‘e’ and spacebar chatter before ‘scroll lock’ does.
• Factory debounce set too low — some boards ship with ~5ms windows tuned for gaming latency claims; fine when new, marginal after a year of wear.
• Contamination — dust, crumbs, or dried drink residue changes the contact geometry. Post-spill chatter on a cluster of keys is a classic.
• Hot-swap socket issues — a barely-seated switch pin can make intermittent contact that looks exactly like bounce.

Confirm it in the tester (30 seconds)

Open the keyboard tester, click it once to arm, and press the suspect key once — deliberately, ten times. A healthy switch lights once per press. A chattering switch double-flashes: two events in the press history a few milliseconds apart, from a single physical press. The held-key chips and raw readout make it unmissable — you’ll see repeat: false on both, proving the OS didn’t synthesize the second event.

Two sanity checks while you’re there: if a whole row doubles (same matrix line), suspect the PCB or a spill, not six worn switches; and if the key shows dead intermittently rather than doubled, that’s the dead-key diagnosis path — same ladder, different fault.

The fix ladder, cheapest first

Fix	Cost	Works for
Raise firmware debounce (QMK/VIA DEBOUNCE, vendor software)	Free	Low-debounce boards, early wear
Software debounce filter (e.g. debounce utilities, keyboard remappers)	Free	Any board — masks the second event in software
Keycap off + compressed air into the switch	Free	Dust/crumb chatter
Re-seat the switch (hot-swap boards)	Free	Socket contact issues
Swap the switch (hot-swap: ~2 min; soldered: a soldering job)	$1–5	Worn contacts — the permanent fix
Replace the board	$$$	Membrane/board-level failure, or not worth the solder work

One honest caveat: chatter is progressive. A 5ms debounce bump might buy a switch another year — or another month. If you’re shopping for its replacement anyway, mechanical vs membrane covers what actually differs — including hot-swap sockets, which turn this exact repair into a two-minute job.

Frequently asked questions

Why is my keyboard typing double letters?

Almost always switch chatter: the metal contacts inside the switch bounce for a few milliseconds when pressed, and if the keyboard's firmware debounce window is shorter than the bounce, both contacts register as separate presses. It's the classic aging-mechanical-switch fault — also caused by low factory debounce, dust in the contacts, or liquid residue.

How do I test for switch chatter?

Use a keyboard tester that shows every raw event — on this one, press the suspect key once, firmly, several times in a row. A chattering switch produces two rapid press events for a single physical press: the key flashes twice and the press history shows back-to-back entries milliseconds apart. Type a paragraph into a text field and count the doubles for confirmation.

Can I fix double typing without buying a new keyboard?

Usually yes, in order of effort: raise the debounce time in the keyboard's firmware/software if it's configurable (QMK/VIA exposes it; some vendor tools do too); remove the keycap and blow the switch out with compressed air; on hot-swap boards, swap the switch in two minutes; as a last resort on soldered boards, replace the switch or the board.

Why do only some keys double-type?

Chatter is per-switch wear — the keys you hammer most (E, A, Space, Backspace, game binds) develop bounce first. A single affected key is the signature; if a whole cluster doubles at once after a spill, suspect residue bridging contacts instead.

Is double typing a software problem?

Rarely — but rule it out free first: if the key lights correctly in the tester and only doubles in one application, it's that app or an input tool. Filter Keys and accessibility features can also warp key timing. If the tester itself shows two flashes per press, it's hardware, full stop.