They do use duplicate keys.
I was with a friend when his dad unlocked and started someone else's car (same colour and it was parked a few spots away) before realizing what had happened.
Turns out the actual owner of that car saw us getting in and was understandably freaking out and came running over to her car screaming and pounding on the windows thinking it was being stolen.
Simply put, they are duplicated. That goes for really any key.
However there are enough combinations that it's virtually impossible to find a twin lock that your key will work in.
If you own a 1997 Honda Accord, and tried your key in every other 1997 Honda Accord you came across, you'd eventually find one that you can open and maybe even start.
Years ago I remember a news story where a lady accidentally drove off in another person's car because it was the same make, model, and color, it was parked near her car, and her key just happened to fit it.
That was a one-in-a-billion fluke, though.
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Even a pretty small car manufacturer produces half a million cars per year.
I checked the key manufacture number on my car keys. It has a five digit number. That means that if they produce 500 000 cars per year, they issue the same keys five times in that year.
But, if you sell the cars all over the world, what are the odds that two cars with identical keys will end up in the same parking lot?
Slim. Very slim.
It still happens, though. And that is kind of hilarious, don't you think?
Anyway. Modern cars don't just rely on the physical key any more. They also have an electronic feature that is called an *immobilizer*. Technically, that is a software feature in one of the onboard computers that requires that the right key is present in the car. Not just the physical key, but also a unique piece of electronics that is manufactured separately from the physical key, that is hidden in the key handle. That the car is then specifically told to pay attention to, once for each key set you get when you buy the car.
My car is keyless. There is a physical key inside the key fob that I can extract by pressing a button, pulling and whatnot. But it's only use is to open the drivers door in case of an emergency when the battery has run out. There is no other physical key lock on the car.
The car responds to the presence of the key fob. A radio signal. Where the key identifies itself to the car. And it's the actual presence of the radio transmitter inside the car that allows me to ignite the engine and drive off.
For fun, I guess, the key fob also has buttons that are able to unlock the doors. And that is yet another radio system.
So, technically my car uses three different systems. One to open the door in case of an emergency. One to start the car and use the keyless lock/unlock feature. And one to open and lock the car from a distance.
It's only the physical key that is the issue here. The others are a lot easier to manufacture with a unique identity.