I think people are just really confused here.
People DO NOT WANT a bunch of same types on the team.
It's irrelevant that Ash has had overlapping types, when it comes to personal desire.
So no it's not that Ash can't have 3 dragon types or 3 fighting types. It's just that no ONE wants that. Well okay excluding those who want specific Pokemon on Ash's team that JUST so happens to be a 3rd shared type or 2nd type or 4th, 5th 6th.
People want variety. As long as it brings something new it can be on the team even if it overlaps.
No one could possibly want Ash with something like Machamp, Sirfetch'd, Primeape, Mienshao, Hariyama, Hitmon, Grapploct, Conkeldurr all on his team at once. That's the point. People want variety of types. There's 18 types, there's no reason to give him 3 of one type unless they all have completely different dual types.
So if Ash gets Flygon, sure I get wanting Dragapault but it's literally not going to give Ash anything new. Sure it would only be his 2nd ghost type, and I can see that being an argument, but compare that to Chandelure in which it actually offers a new type on the squad with fire type.
People are going to have their preferences, and so I don't understand is the insistent argument "Well Ash can totally have this because type overlapping doesn't matter" okay that's obviously true, but what's your point? The person's point is they don't WANT another of that type unless it truly brings something new or at least marginally different.
Like take Dragonite and Lucario. And then Corviknight, Corviknight should technically offer Ash something relatively more fresh than Dragapault would (if Ash gets Flygon) because in the moment, Ash would only have one flying and one steel, so having a Pokemon bump that to two flying and two steel would be better than three dragon and two ghost especially when there are just ultimately typed Pokemon out there.
The only types that could possibly not matter at the moment, is fairy and psychic, because its unlikely Mr. Mime will be in a PWC so Ash could get arguably three fairy or three psychic types if we INCLUDE Mr. Mime (but of differing types), heck Ash could straight up get a psychic/fairy, and just not have Mr. Mime not have to deal with battling.