Yes, they should be able to tell the difference between grit and oyster shell.
If they have had grit available for a while, then they should not be trying to eat oyster shell instead of grit.
Yes, I would definitely check that before worrying.
I agree with
@aart that gorging on the oyster shell could be a problem, but I would make sure before worrying or taking it away again. If they are actually spilling most of it, then they are not overeating from it. Spilled oyster shell will not hurt the chickens or the bedding or any compost pile you may eventually put the bedding in, but of course it's inconvenient for you because you have to keep refilling the container and you have to buy more sooner.