I know what you're thinking... and I still can't believe it, but...

This morning, our database flagged a duplicate UUID (v4). I checked, thinking it may have been a double-insert bug or something, but no.

The original UUID was from a record added in 2025 (about a year ago), and today the system inserted a new document with a fresh UUIDv4 and it came up with the exact same one:

b6133fd6-70fe-4fe3-bed6-8ca8fc9386cd

We're using this: https://www.npmjs.com/package/uuid

I thought this is technically impossible, and it will never happen, and since we're not modifying the UUIDs in any way, I really wonder how that.... is possible!? We're literally only calling:

import { v4 as uuidv4 } from "uuid";

const document_id = uuidv4();

... and then insert into the database, that's it.

Additionally, the database only has about 15.000 records, and now one collision. Statistically... impossible.

Has that ever happened to anyone?! What in the...

Funny story no one will believe, but it’s true. A good friend of mine joined a startup as CTO 10 years ago, high growth phase, maybe 200 devs… In his first week he discovered the company had a microservice for generating new UUIDs. One endpoint with its own dedicated team of 3 engineers …including a database guy (the plot thickens). Other teams were instructed to call this service every time they needed a new ‘safe’ UUID. My pal asked wtf. It turned out this service had its own DB to store every previously issued UUID. Requests were handled as follows: it would generate a UUID, then ‘validate’ it by checking its own database to ensure the newly generated UUID didn’t match any previously generated UUIDs, then insert it, then return it to the client. Peace of mind I guess. The team had its own kanban board and sprints.

What you're talking about is so extremely rare that it's much more likely that the entire Earth is destroyed by an asteroid right this inst...

> I thought this is technically impossible

Actually it's not impossible, but very very improbable.

P.S. You should play a lottery/powerball ticket

P.P.S. Whenever I use the word improbable, the https://hitchhikers.fandom.com/wiki/Infinite_Improbability_D... comes in mind

Please, do not use b6133fd6-70fe-4fe3-bed6-8ca8fc9386cd, I checked my database and I was using it already.

It's not happening by chance, there is a bug somewhere.

From what I skimmed the package should just call to the js runtime's crypto.randomUUID(). I think it should always be properly seeded.

I think it is extremely unlikely that the runtime has a bug here, but who knows? What js runtime do you use?

Reminds me of some code I saw running in production. Every time we added a new entry, we were pulling all the UUIDs from this table, generating a new UUID, and checking for collisions up to 10 times.

Poorly seeded prng.

The math says no. UUID v4 has 122 bits of randomness, so collision probability for 15K records is N²/(2·2^122) ≈ 2·10^-29. That's somewhere around "fewer collisions per universe lifetime than atoms in your liver." Whatever you're seeing, the culprit is overwhelmingly somewhere else.

Things to check, in descending order of how likely they actually are:

1. Data import / migration / backup restore, perhaps? Did anyone load a CSV, run a seed script, restore a snapshot, or copy rows between environments at any point in the last year? This is what "duplicate UUID" is in 99% of cases. Check git on migrations, ops history on the DB, and ask anyone who might have been moving data around.

2. Application retry / rollback bug maybe? Code path that generates a UUID, attempts insert, fails on constraint violation, retries with the same UUID variable still in scope. Check whether UUID generation lives inside or outside the retry boundary.

3. Older versions of the uuid package in certain bundler environments would fall back to Math.random() instead of crypto.getRandomValues(). What version are you on? Anything <4.x is suspect; modern v8+/v9+ uses crypto everywhere correctly.

4. Could also be a process fork bug. If a UUID generator runs in a child process spawned from a parent that already used the PRNG, the entropy state can get copied. Rare in Node specifically, more historical in old Python/Ruby setups.

If you've ruled all of those out and the row really was generated independently a year apart via crypto.getRandomValues, go buy a lottery ticket. But it's almost certainly cause #1.

Buy some lava lamps

Buy a lottery ticket

I fully agree. It makes no sense. Yet...

The only guesses I'm having is that we originally generated UUIDv4s on a user's phone before sending it to the database, and the UUID generated this morning that collided was created on an Ubuntu server.

I don't fully know how UUIDv4s are generated and what (if anything) about the machine it's being generated on is part of the algorithm, but that's really the only change I can think of, that it used to generated on-device by users, and for many months now, has moved to being generated on server.

1 in 4.72 × 10²⁸

1 in 47.3 octillion.

i'd be suspecting a race condition or some other naive mistake, otherwise id be stocking up on lottery tickets.

(lol at the other user posting at the same time about the lottery ticket.. great minds and all that.)

Would the UUID v7 be more collision proof? Hard to say because it takes time into account but then the number of entropy bits are reduced hence the UUID generated exactly at the same time have more chance of a collusion because number of entropy bits are a much smaller space hence could result in collusions more easily.

Thoughts?

Why not to have timestamp-uuid instead ?

Just a stupid question, but why not append the date, even in seconds as hex. It's just a few bytes and would guarantee that everything OK now will be OK in the future?

The chance of a UUIDv4 collision is very low, but it is never zero.

If everything is done properly, then this is very likely the one and only time anyone involved in the telling or reading of this account will ever experience this.