This section is from the book "The Law Of Contracts", by William Herbert Page. Also available from Amazon: Commercial Contracts: A Practical Guide to Deals, Contracts, Agreements and Promises.
A later contract which is invalid and unenforceable does not abrogate an earlier contract though it was intended so to do. If the second contract is voidable as by reason of duress,1 and the second contract is avoided by reason of such defect, it will not operate to abrogate or modify the earlier contract. So if the second contract is voidable because it is entered into with an unauthorized agent of the adversary party,2 it will not, if avoided on that ground, abrogate or modify the earlier contract. A public contract which by statute must be let to the lowest bidder on advertisement for bids, cannot be modified in a substantial element after it has once been let.3 So if a later contract is unenforceable because of the Statute of Frauds it cannot, if attacked on that ground, abrogate or modify an earlier contract.4