Remote controls

What keeps remote automotive door lock frequencies to be different enough so that there are no duplicates?

The frequencies don’t matter. The car and fob generate a list of random numbers (256 40-bit numbers I think) and sync the list. The fob sends a random number and the car checks it against the list.

See this: http://auto.howstuffworks.com/remote-entry.htm

Garage door openers work the same way. Unless you have one built in 1955.