For cards like Bunker's Grenade Launcher or AZs Hoarfrost, when you are told to pick a taget then choose a second target, can the second target be the same as he first target? Or are you obligated to have 2 different targets?
Anytime it says to pick a second target it must be a brand new target. Though say in AZ's case he could hit himself with the fire damage thus hitting a target for cold. Then the second target could be that target that was hit with the cold since technically AZ was the first target.
Also any redirection would allow you to hit the same target more than once. Such as Bunker's Grenade launcher, it could hit 1 enemy target, then hit Mr. Fixer who happens to have driving mantis in play, thus redirecting it to the target that was hit by the first one. Or even Mr. Fixer with Driving Mantis and Dual Crowbars and no other buffs could hit a enemy then himself and redirecting to the same enemy.
I think about the only time when you can use a multi-target effect on the same target twice is the Adept's Scherzo of Frost and Flame song, which basically says "Hit one target for one point of fire damage. Hit one target for one point of cold damage." - it doesn't specify that the targets need to be different so in this case you can hit the same thing twice if you want.
But yes, I like being able to squeeze extra damage (or, in Ab'Zero's case, a free heal) out of a multi-target attack by hitting a fellow hero. Mr Fixer (with Driving Mantis out) as mentioned, or Nightmist with her Amulet, or Ab'Zero when he has both modules out and you whack him with fire damage (obviously you're likely to be playing Ra when you do this) so he can then either heal or hit someone else with cold damage :).