From the rulebook:
“While you can use heroes and villains together, or characters from different teams, you can’t use two cards with the same name (even if they have different subtitles)”
Also, even though some character dice may have the same image and stats between different sets, it is generally not acceptable to use dice that do not match the card/set in a tournament setting. This is determined by comparing the die to the images at the bottom of the card to make sure all colors, symbols, and stats match. If you are playing casually and your opponent is comfortable with you using a mismatched die (generally a different color with the same stats) you may use a substitute die.