As a general rule, money paid pursuant to a settlement agreement is gone for good. The bar for duress is pretty high--specifically, it usually (always?) requires a threat of bodily harm.

And, even if you could somehow prove a settlement agreement to be unenforceable, what would be the point? For one thing, you'd cancel the release of liability you obtained. And secondly, you already performed your part of the agreement (paying the money). In general, I wouldn't see any benefit in this course of action.