Justice League (Silver Age)
Once during each game, choose any team ability or opposing character. Each friendly character using this team ability modifies its attack value by +1 when attacking the chosen character or a character possessing the chosen team ability.

Ok so this came up in a game I played tonight. It says an opposing character. Say someone was playing multiples of the same character, would the bonus only apply to one of them or all of them?

Just one. If your opponent is using 2 of the common Batman, you don't say, "I'm choosing every common Batman;" you say, "I'm choosing that common Batman" and point to one.

But if you pick the team ability instead, you can get the bonus against entire teams in some cases.

The only time I believe it works for multiple characters is if you pick the TA option.

I hadn't really ever thought about it but say you pick mystics and you double shot 2 mystics, you just get +1 right? That's the way I've always played it but re-reading it now it does say "when" not "if" and if there were multiples I'm not sure why it wouldn't stack with itself...

The TA only applies once towards each figure regardless of how many qualifying figures you're targeting. Also, if you are targeting two or more figures but only one of them qualifies for the TA bonus, you'll have a +1 towards that figure only and not the entire roll.