If you "double" a creature's power more than once in a turn, each doubling effect applies independently. For example, suppose Junk Jet is attached to Infesting Radroach, a 2/2 creature. If you activate Junk Jet's second ability, Infesting Radroach will get +2/+0 when that ability resolves, making it a 4/2 creature. Doubling its power again will give it +4/+0, making it an 8/2 creature.