I don't think it was the first time that any of Ash's pokemon used Double Team. I'm pretty sure that he told Pikachu to use Double team in Team Rocket's van in the episode when Ash first meets Richie.
As for the Golden Swellow part, my theory is that the energy was channelled through Swellow to increase his attack power. Think of it as an "electrical Airial Ace."
EDIT: Never mind, that was an agility.