well, at least with the different coronavirus strains, they are similar enough that getting one is effectively like an immunisation against all the other strain

well, at least with the different coronavirus strains, they are similar enough that getting one is effectively like an immunisation against all the other strains. Now you could probably get two strains simultaneously if you were independently infected separately by both at the same time. Delta, however, isn't quite common enough for that to be likely I think.