Final answer:
Promising to perform a preexisting legal obligation is referred to as a 'Preexisting Duty' and is not considered valid consideration in contract law. In determining which obligation to fulfill when duties conflict, one should use judgment to assess which is more pressing, following the principles of prima facie duties as outlined by philosopher W.D. Ross.
Step-by-step explanation:
The term you're looking for when one promises to do something that they already have a legal obligation to do is Preexisting Duty. This is not considered a valid consideration in the context of contract law because it is a duty that the person is already bound to by law. In contract law, for a new agreement to be binding, there must be a fresh consideration - something of value exchanged - which is absent when one simply commits to doing something they are already legally required to do.
The Role of Judgment in Resolving Conflicts between Duties
As per Ross's theory of moral commitments, when faced with conflicting duties such as the duty of fidelity (to keep a promise) and a duty of non-maleficence (to not harm others, which could include helping someone in an emergency), an individual should use judgment to decide which duty is more pressing and ought to be fulfilled.