battling the same pokemon shouldnt and probably and hopefully wont happen.
We most likely wont battle the same exact Pokemon for anyone, but, we most likely, and will be battling some same Pokemon for most likely everyone. Elesa has Emolga and Zebstrika based on the promo, I'd assume the Gym Leaders and Elite Four who stay from the last game will have 1 or two changes, but not their whole team.
I'd have a issue with battling the same Pokemon, there probably will be rematches, you can battle some more new additions then.
In terms of gyms, I see it staying how it was in terms of numbers:
1st gym-2 Pokemon confirmed. Patrat(Level 11) confirmed. Not sure on Cheren's second Pokemon, I'm still waiting to know if his text was translated or not to know if what he said with Patrat would give a hint to which one it is. Regardless, I see him with Lillipup(Level 13 if it's his final Pokemon, Level 9 if it's his first Pokemon), maybe Herdier(Level 13), because he's got a Stoutland in the promo, plus I'm sure it'll be seen before battling him, which fits with the format of the Black and White Regional Dex which I assume will be the same this time around as well. I'm certain Work Up will be given as the TM.
2nd gym-2 Pokemon confirmed. Koffing confirmed, I'd hope for a level of 18-22. I'm going to say she'll have a Whirlipede as her final Pokemon(Level 20-22, depending on Koffing's level). Venoshock I'm certain will be the TM.
3rd gym-Nothing known aside from a small little outline of the badge which only matches Burgh's Insect Badge, but possibly could be Shizui's, but I'm leaning more to it being the Insect Badge and I don't see him leaving. Three Pokemon for him again I'd say, keeping Dwebble(Level depends on Homika's Pokemon, around low to mid 20's though), and Leavanny(mid 20's for level), replaces Whirlipede for something new(Well...old...er non-fifth generation Pokemon), not sure what but preferably something lesser used, will be same level as Dwebble. Same TM.
4th gym-Nothing known, I'm going to assume it's Elesa's though. Three Pokemon still, of which are Emolga(Level 28), Flaaffy(Level 28), and Zebstrika(Level 30), Flaaffy who is confirmed(Level I believe is 28) and Emolga and Zebstrika who are likely confirmed based on the promo. Same TM.
5th gym-Nothing known, I'll say it's Clay though. Again, three Pokemon, I'll say he'll keep Excadrill, not sure on Krokorok and Palpitoad, replaces one of them for a non-fifth generation Pokemon, I'd assume the levels would be mid 30's. Same TM.
6th gym-Nothing known, I'm thinking Skyla. Three Pokemon again, keeping I don't know, replacing I don't know, maybe she can have two replaces on her team, levels being high 30's. Same TM.
7th gym-Nothing known, I'd say it's Shizui. Three Pokemon for him is confirmed. Not going to bother trying to figure out what he may have, levels low 40's to mid 40's. Scald for the TM I would assume.
8th gym-Nothing known, I believe it will be Iris and Drayden again, only switched this time. Three Pokemon for both, replaces Haxorus, both have the same Pokemon as eachother aside from a few differences(Gender and Ability), levels end with high 40's.
Elite Four-4 Pokemon each again, same format as before, same members as before, everyone replaces two Pokemon for non-fifth generation Pokemon. Levels are Ended at 55 or so.
Champion-I'm thinking Alder is still champion, changes most of his team, levels end at like level 60.