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: How Remote Entry Works | HowStuffWorks
Garage door openers work the same way. Unless you have one built in 1955.