Can you fight the Doppelganger twice or is it a one time thing?

and

For Bikimin's Flyers is there like a list where people are at times are is it always random. I need like 4 more people and its really annoying not finding them.

I don't know about the Bickman quest, but as for the doppelganger, you can fight him twice. One time during the main game, and the second time after you beat the game.

I think people always show up on the same "place".. For example 1/2 and blu3 are always around Mac Anu (doesnt matter in what district you are).. The only place I know that spawns different people everytime you go in/out is the warp gate hall in Mac Anu

a good way to get most of the flyers is to fight in an arena battle and when you get out there will usually be one or two new people untill there are none left to get in that city.