Dude two perfect attacking Gyarados? That's awesome! I wish I had one.
I have four perfect attacking Jolteon's, but I only fully maxed out one of each offensive moveset (so two are level 20).
I would not fully power up your second carbon copy Gyarados. Stardust is so hard to come by man, and at higher levels you will regret having to maintain two of the exact same mons. I'd keep the second at its current level and use it when you can/keep it as a trophy.