The federal government increasingly has spent potentially duplicative funds on individuals receiving healthcare benefits through the Veterans Administration and Medicare Advantage, a new analysis finds.

Between 2004 and 2009, the government spent $13 billion on federally funded care for individuals covered by both Medicare Advantage and VA benefits, according to Brown University investigators. With enrollment in both government programs growing ever year, the study’s authors found the potential for duplicative costs to be “vexing” during a time of “severe financial pressure” on Medicare. They found that the biggest chunk of this spending was outpatient care, followed by acute and post-acute inpatient care, then prescription drugs. Medicare Advantage pays for skilled nursing care following an acute-care episode.

According to the analysis, if Medicare Advantage enrollees receive “Medicare-covered services from another federally-funded hospital or other healthcare facility, and this facility cannot be reimbursed, then the government has made two payments for the same service. In this scenario, private Medicare plans receive taxpayer-funded subsidies to insure veterans who in turn use another government-funded health system to receive medical care.”

The analysis was published online Tuesday in the Journal of the American Medical Association.