In estate, will and trust litigation, duress and undue influence are sometimes alleged together. However, while they overlap in some areas, they are distinct legal theories.
To claim duress, a person must be oppressed by the wrongful conduct of another. In re Estate of McKenna, 500 S.W.3d 850, 860 (Mo. Ct. App. 2016). It is never duress to do what a party has a legal right to do. Id. Undue influence, on the other hand, does not require wrongfulness or illegality. See Security Savings Bank v. Kellems 274 S.W.112, 114 (Mo. Ct. App. 1925) (it is not duress to institute or threaten to file lawsuit based on what one views to be his/her legal rights).
As a general matter, then, most viable claims of duress are also undue influence claims, whereas an undue influence claim is not necessarily a duress claim.