A couple games ago we were playing with a 
and she Q's a fleeing enemy, and right after it hit, he flashed away and surprisingly it hit him with the bounce as well.. and killed him. I would've posted this in "Report a bug", but it's not exactly a bug. I do think her Q should only be able to hit an enemy once though.
That's actually pretty hilarious and doesn't sound like a bug based on the wording of the ability.
"Fires a shot at a target unit, then bounces and to hit another unit behind it"
It would be REALLY hard to time a flash to get hit by it twice.
It's not really a bug, the power is working as programed. Double Up hits the first target and then tries to bounce to a second target; what's happening in that case with Flash is that it's hitting the first target, and when it begins its calculations and trajectory to find the second target, the opponent has Flashed into the zone where the second bounce is calculating and getting tagged as a viable second target for the bounce. Mathematically speaking, this is logical and it fits all requirements for Double Up; Even in a real life standpoint, this also makes sense should someone develop short range teleportation technology and manage to hit the same projectile twice if it warped into the path after being struck already. The only problem I can see is that in a balance perspective, this may make Miss Fortune a bit too strong, and it essentially invalidates one of the most common Summoner spells used as a means of escape.
Another unit can mean the same unit in range because it then becomes another unit
I think this is can happen and i think it actually happened to me in-game, but this only happened once...maybe you can test it in a custom game to see.
