MPM is not doubled. MPMs apply to each way. With a MPM at 20550, you need to be under on each way to avoid surcharge or fare break. 22000 on one way and 17000 on the way back will lead to a 15% surcharge for the first fare component, even if the total mileage is less than the doubled MPM. It is also not possible to fly 26000 miles one way and 14000 on the way back.

Edit: the previous post has been published while this post was written