dnd 5e: What happens when Polymorph throws himself at a creature already affected by Polymorph?

The rules for combining magical effects are found in the Player manual, page 205. (Note that the last line was entered by typos).

The effects of different spells add up while the durations of those spells overlap. However, the effects of the same spell cast several times are not combined. Instead, the most powerful effect, such as the highest bonus, of those pitches is applied while their durations overlap.
Or the most recent effect is applied if the throws are equally powerful and their durations overlap.

So, if the two throws are equally powerful, the last one is applied, which means that the creature takes that form. Because the previous spell does not end with the new spell, it can be reverted to the first spell. polymorphic organism I should finish the second.

However, the power is not defined by the game and, therefore, will depend on the DM award. The spell that is cast at a higher level will probably be interpreted as a more powerful version (I would certainly say so). But how polymorphic organism It has no innate benefits from being on top, you may not see it at higher levels very often.