Simple RAW for 5e V: The Disadvantage Strikes Back

[Zhorn]

A163
no stacking you only get the one double

If a circumstance suggests that your proficiency bonus applies more than once to the same roll or that it should be multiplied more than once, you nevertheless add it only once, multiply it only once, and halve it only once.

and again

If a circumstance suggests that your proficiency bonus applies more than once to the same roll, you still add it only once and multiply or divide it only once.