>Each of the trainers is completely random, selecting multiple Pokémon at random.
It is possible to rebattle the same trainer again, but it is unlikely, as the trainers are random.
However, according to this page on Bulbapedia,
>Any Pokémon Breeder will own two members of the Chansey evolution family.
This means that it doesn't matter if you get the same trainer to grind levels because any Pokemon Breeder trainer will have two members of the Chansey family.
Hope I helped!