There are no search results in the FAR for "extended overhead", so that is not a term used in the FAR. FAR part 31 helps with understanding cost elements like overhead and profit. As to the applicablity of the cost principles in FAR part 31 for the action you are working, review FAR subpart 31.1. FAR subpart 31.2 helps to understand allowability, reasonableness and allocability of costs as well as when costs are unallowable. FAR 31.202 explains direct costs and 31.203 explains indirect costs. In no case can a contractor charge for the same cost twice. If there is a cognizant DCAA office for the contract in question, it is recommended to reach out to your DCAA POC as they will know the intricacies of the contractor's accounting system and charging of costs.