26 CFR 1.1474-4: -4 Tax paid only once.

(a) Tax paid. If the tax required to be withheld under chapter 4 on a payment is paid by the payee, beneficial owner, or the withholding agent, it shall not be re-collected from any other, regardless of the original liability therefor. However, this section does not relieve a person that was required to, but did not, withhold tax from liability for interest or any penalties or additions to tax otherwise applicable.

(b) Effective/applicability date. This section applies January 28, 2013.