I'm not sure that you can't play
7R152 then play
15R86. This is what the comprehensive rule says about uniqueness:
When a unique card is in play and currently active, you cannot play another card that has the same title.
If you play
7R152 first, you do not have a unique
Mumak Commander in play and active, so it would surely appear that you can play another card with that title. If, however, you played
15R86 first, then you clearly would not be able to then play
7R152 because they have the same title.
Assuming that's true, then yes you could exert
7R152 twice using the ability on
15R86 because the latter says 'Exert
Mumak Commander twice' which includes any card with the title '
Mumak Commander'.
Thranduil