Rep. Vicky Hartzler, R-Mo., said Thursday she's worried the military is paying twice for some of its capabilities overseas because troop number caps force it to use contractors for certain job functions.

The Obama administration has capped the number of U.S. troops in Iraq at 5,262, while the cap in Afghanistan is set to drawdown to 8,448 by January. The military has said it does not expect to lose many capabilities in Afghanistan because the reduction on boots on the ground will mainly come from putting administrative service members outside of the country and from hiring contractors.

As a result, troops with certain skills must stay home to keep the total number of service members in the country under the troop cap, and contractors are instead hired for these roles. One example used by Hartzler in a House Armed Services Oversight and Investigations Subcommittee hearing she chaired on Thursday was aircraft maintainers. Service members who work on aircraft must stay at home, letting their skills atrophy, while the military pays a contractor to do the same job.

"We're paying multiple ways through this scenario. It's very concerning," she said.

Hartzler said she's asked the Pentagon how much it is costing taxpayers to backfill these positions, but has not received an answer. Cary Russell, the director of military operations and warfighter support at the Government Accountability Office, said he has not looked specifically at these costs.

But retired Army Lt. Gen. James Dubik, a senior fellow at the Institute for the Study of War, estimated that it takes 400 to 500 civilian contractors at a cost of about $100 million a year to work on a brigade's worth of aircraft.

It's unclear how many U.S. aircraft are currently in Afghanistan, or how that estimate may help determine a cost.