The thing is though that every time Ash has shared a Pokemon with a rival/recurring character, his Pokemon either didn't fully evolve or the rivals/recurring characters Pokemon didn't show up anymore after Ash got his fully evolved (And you can potentially make the same argument for the Pokemon owned by his travelling companions and their rivals - Kenny had Empoleon, Dawn kept Piplup, Aria had Delphox, Serena had Braixen)
Paul: Torterra and Gliscor didn't show up anymore the moment Ash got his
Bianca: Ash got Pignite, Bianca got Emboar
Trip: Snivy stayed unevolved, while Trip got Serperior. The moment Ash got Tranquil, Trips didn't show up anymore.
The idiot had Samurott, Ash had Oshawott (And Burgundy had Dewott)
Sanpei: Has a Greninja, however, the moment Ash got his, Sanpei didn't show up anymore.
Ash had Torracat until the league, but after Ash got his Incineroar, Kukui's didn't show up on-screen anymore.
And while Ash shared Lycanroc with Gladion, there was a form difference between them, so they could be distinct (Same with his and Mays Bulbasaur).
This is way i can't see Ash ever getting the same Pokemon Leon has. Unless he already owns it, like Charizard or their is a form difference (Shiny or otherwise).