About three years ago, Congress granted agencies wide authority to rehire federal retirees and pay them both their full pensions as well as their full salaries. But the Government Accountability Office today released a report that found six federal agencies are barely using the so-called dual compensation waiver authority at all. GAO looked at how many waivers were granted by the Treasury Department, Office of Personnel Management, U.S. Postal Service, U.S. Agency for International Development, Small Business Administration, and the Nuclear Regulatory Commission in fiscal 2010 and 2011. Those six agencies granted only 187 waivers in the first year, and…