Can or should an IDIQ contract be issued for services that are also provided under a pre-existing master cost-sharing agreement in the same area?
This question appears to be specific to the terms of the master agreement contract, to which we are not privy and on which we cannot comment. You rightly recognize that a requirements contract requires the Government to purchase all of its requirements for the specified item or services up to the maximum quantity in the contract. As such, the Government is not allowed to purchase those services elsewhere. Here, it appears the customer is arguing that the terms of the master contract would nonetheless allow issuance of a separate requirements contract.
You need the advice of the cognizant contracting officer and legal counsel to answer this question.