I'm kinda sad nobody else is posting their reasons. I want to see a counterargument to my former post, as I don't feel I understand Arakune's abilities to the fullest. Oh well, from experience I know fighting game posts don't get much attention this predominantly FPS forum. Who do you side with, op? :)

Batman with prep time.

Nvm, looked it up. At their core, both Double, and Arakune seem to be shapeshifters. Arakune seems to have a good amount of natural abilities, such as poisonous insectsat his command, the ability to curse his opponents, and the ability to fire balls of concentrated energy. Double on the other hand, doesn't have any such abilities on her own, but seems to be able to copy the abilities of others perfectly. The two most odd examples of this are being able to fire Parasoul's gun, and make Leviathan's fire, both presumably made of her flesh. She also has shown some short range teleportation abilities, with her "Nightmare Legion" attack. Edit: not to mention her ability to copy teleportation moves, like Peacock's "The Hole Idea". Double's "replication" abilities are the reason, I think, that she would win, as anything Arakune could throw at her, she could throw right back, in addition to the powers of others. That being said, Double has been shown to have a weakness to energy attacks, as Annie's Galaxy Cutter, and Squigly's Inferno of Leviathan both presumably killed her instantly, and if Arakune's energy attack is strong enough he could do the same. But Aeon has foretold our victory. Double gets my vote.