Is collateral estoppel or res judicata a defense?

The appellees argue that appellants' claims are barred both by collateral estoppel (issue preclusion) and res judicata (claim preclusion). Collateral estoppel and res judicata are related but distinct doctrines that may bar a party from litigating certain issues or claims in a subsequent proceeding. Both are affirmative defenses. See Leather v. Eyck, 180 F.3d 420, 424 (2d Cir. 1999).

Collateral estoppel "applies when (1) the issues in both proceedings are identical, (2) the issue in the prior proceeding was actually litigated and actually decided, (3) there was [a] full and fair opportunity to litigate in the prior proceeding, and (4) the issue previously litigated was necessary to support a valid and final judgment on the merits." United States v. Hussein, 178 F.3d 125, 129 (2d Cir. 1999) (internal quotation marks and citation omitted).