Eight ball, corner pocket
If it states 5 targets, may you choose one target 5 times or not?
Often it depends on the exact wording. If it says "Deals up to 5 targets X damage", for instance, you have to choose up to 5 separate targets. Most of the multiple-target damage in the game is like this.
If it says "[You] hit one target for 2 melee damage. You hit one target for 1 toxic damage.", then you can choose the same target both times.
As kind of a rule of thumb, if you have not started a new sentence on the card, then you need to keep choosing separate targets. Remember, though, that you can often choose in what order you hit those targets, so you can get some interesting pinball effects.
Hope that helps.