What happens when a caster casts Polymorph on a creature that is already Polymorphed by a different caster?
It takes the form of the last polymorph, unless the first was upcast
The rules for combining magical effects are found in the Player's Handbook, page 205. (Note that the last line was introduced by errata.)
The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don't combine, however. Instead, the most potent effect—such as the highest bonus—from those castings applies while their durations overlap. Or the most recent effect applies if the castings are equally potent and their durations overlap.
So, if the two casting are equally potent, the last one applies, meaning the creature takes that form. Because the old spell isn't ended by the new spell, it can be reverted to the form of the first polymorph should the second one end.
However potency isn't defined by the game and so will depend on DM adjudication. The spell being cast at a higher level would probably be interpreted as a more potent version (I would certainly rule so). But as polymorph has no innate benefits from being upcast, you might not see it cast at higher levels very often.