If my maths is right, assuming that the algorithm for deciding the setup is completely random, there exists a 1/960 chance for a desired FRC setup to appear. Note the word 'desired' - let's say for sake of hyperbole that you wanted the QRKBBNRN setup.
For that setup to appear twice, that's a 1/960 * 1/960 chance, or a 1.08507*10^-6% chance.
But, for any arbitrary position to appear twice, it's a 1/960 chance; the probability of you getting said arbitrary position in the first place is 1, because when you play FRC, the initial position always has to be determined, whether it's done for you or not. If you roll 2D6, the odds of you rolling your desired number twice is 1/36, but the chance of rolling any number twice is 1/6.
Two games started more or les simultaneously and they apear to have the same starting position.
https://www.chess.com/daily/game/178100402
https://www.chess.com/daily/game/178100416
Is this a coincidence? Makes me wonder how the starting position is chosen, and if there is an option to choose the position.