THE LEGAL WATCH
Trusted Contract Advisory
TLW - Contract Awareness Initiative
TLW - Legal Awareness Initiative
Essentials for a valid consideration are-
1 it should be at the desire of the promisor. If the other party is voluntarily giving something than it is not considered consideration.
2 it can be given by the promisee (the person whom to which a offer is being made) or any third person on behalf of the promisee
3it can be past, present or future
4there should be some act or promise by the promisee which constitues a consideration. If nothing is done in exchange of the promise i.e., if there is no act than there is no consideration.
Coming to your 2nd question i.e., does a pre existing duty constitues a consideration or not.. The answer is...
Performance of an existing legal duty, or contractual duty is no consideration.
Hope that works..