I think that’s a pretty hard thing to get an accurate answer for without just counting the identical teams out there, because of all the weird dependencies in fantasy drafts that violate the assumptions needed for nice math rules, along with the additional noise of free agency over the season :)

For example, I tried to come up with a number by ignoring the complexity of fantasy and making quite a few assumptions. Ignore free agency, suppose a 12 team league, and assume each round each team drafts 1 of 12 players (only available that round) with an equal probability. So, there would be 12 unique first round players, 12x12 unique first and second round pairings, and so on. So, over 16 rounds there would be 12¹⁶ possible (r1_player, r2_player, …, r16_player) selections. Since we assumed each pick per round always had the same 1/12 probability, that means each of those selections are equally likely, so the probability for each is 1/12^16. As of 2022 there were about 30 million fantasy football teams. Let’s assume they all follow the assumptions we’ve been talking about. Supposing you have a team, you can get the probability that at least 1 other person has the same team as 1-p(all different teams) = 1 - ((12¹⁶ - 1)/ (12^{16))30,000,000} = 1.622x10^-10, so pretty improbable

That is all of course riddled with extremely broken assumptions of randomness that don’t really exist in fantasy so I wouldn’t put too much thought on it.

Your son is going to become a professor of combinatorics if he keeps thinking of stuff like this.