Wizkids Rules!

"If you have a Villain fielded, pay [1 Shield] to give a target character -1A and -1D (knock out those with zero defense)."

Most trigger abilities seem to have discrete triggers that can occur multiple times in a turn.

Doctor Doom reads like a trigger ability (If/when X, pay Y to get effect Z), but the 'trigger' (if you have a Villain fielded) cannot trigger multiple times in a turn.

So is the if statement here describing a trigger that allows for 0 or 1 uses on a turn or simply there as a check of the game state, similar to "if active" abilities?

This Global is best described as conditional. If you have a Villain in the Field Zone, you may use Dr. Doom's Global. You may use the Global as many times as you can pay for it, as long as there is a valid target.

In the future, this ability would probably be worded:

If you have a Villain active...

Since having 1 or more Villains nets the same effect.

-The Dice Masters Rules Team.