When a card says up to x targets (for example expatriette's assault rifle) can you target the same target multiple times as long as it doesn't die? (going with the ex. can you use expatriette's assault rifle to deal one card 6 damage?)
Nope, you can only hit a particular target once.
There are ways to deal that damage to a single target, but they generally involve redirection shenanigans. For instance, if Mr. Fixer has both Dual Crowbars and Driving Mantis, he can hit one target for 2 damage, then hit himself for 2 damage but redirect that second instance of damage to the same target as the first.
No.
There is one quirk, though. There are some cards which read "Deal one target 1 <type> damage. Deal one target 1 <type> damage." These cards can hit the same target twice. If the second line was "Deal one other target..." it wouldn't work.
A much more abstruse question along these lines. Say Tachyon plays a Sonic Vortex with four Bursts in her trash, and all other heroes are incapacitated, so the only targets that exist are Tachyon, Apostate, and a Relic Spirit, the latter at 6 HP and a single Tome of the Unknowable being the only Relic in the trash . You cannot currently choose four targets for the Vortex, but after you hit the Relic Spirit, and hit yourself but redirect it with Synaptic Interruption to also hit the Relic Spirit, you finish it off, and the Tome of the Unknowable enters play. Can you now choose the Tome as the fourth target of your Sonic Vortex (well, the third, since you still haven't resolved your hit against Apostate)? Or were you "locked into" the targets you had to choose for it, before you started resolving any of the hits?
You can choose the Tome as the new target. You go on choosing targets for as long as you need to, just making sure never to pick a target you had already picked before.
Yup, works well against Gloomy if you whack a cultist with a multi-hit (killing it) and then the resultant Zombie with another hit of the same attack. The destruction of the cultist, in this case, technically interrupts your damage-dealing…so say you were playing Bunker and using a Grenade Launcher, you hit the cultist for the first hit and it dies (because it only had a couple of hp left). It then interrupts you from attacking a second thing to go "Hang on, I have to turn into a zombie now…" and said Zombie comes out of the trash. Then you go "Okay, now where was I…okay I can hit another thing…ooh look, a zombie >:)" and said Zombie goes bye-bye as well. Then I suppose you can plink Gloomy for the other hit (assuming no other viable targets in play of course ;)).
If an attack says "you may deal X targets X damage", then you have a choice as to whether to do the damage at all. If it says "you may deal up to X targets X damage" then you can choose anywhere from zero up to that number of targets. If it says "deal X targets X damage" there is no choice - you must hit that many targets…though I can't think of any multi-hit attacks which force you to do this. You need to be careful with stuff like Mr Chomps, though, who will always attack one thing at the end of Unity's turn. If you're fighting an unflipped Miss Information when there are no environment targets or targettable Diversions in play, you're gonna have to hit a hero target…
End of Days works that way as well.
If destroying a card brings out more targets the End of Days will attempt to destroy them until all valid targets are destroyed, before it attempts to destroy itself.
Yup, I was gonna mention that too actually, but for some reason I didn't. I had to go out right after I sent that message so I might've been rushing a bit at the end.
I knew about EoD, but I wasn't sure whether it could be generalized to things which hit a finite number of targets.