Steadfast Protection: Resistance/Defense: Global
The Defense does stack with any existing Defense you may have. The IO itself can only be slotted once, however, so you won't be stacking multiples of it.
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I believe it's a unique, and it only applies to the person who actually slots the IO, even if it's slotted in an AoE +res power. Therefore, there is no possible way to stack it.
Edit: Mid's shows it as a unique, and that's plenty of verification for me.
It's unique, meaning you can use only one of these on one toon.
That's why people buy the other +3 def enhancement (the pvp one which in practice is less useful since it gives only tp protection instead of the res from Steadfast, tp protection is not useful in PVE) for 3 billion since it's very rare.
The -KB one from the same Steadfast set can stack multiple times tho.
Does Steadfast Protection: Resistance/Defense Global +Def stack?