26 C.F.R. § 1.161-1
Section 161 provides for the allowance as deductions, in computing taxable income under section 63(a), of the items specified in Part VI (section 161 and following), Subchapter B, Chapter 1 of the Code, subject to the exceptions provided in Part IX (section 261 and following), of such Subchapter B, relating to items not deductible. Double deductions are not permitted. Amounts deducted under one provision of the Internal Revenue Code of 1954 cannot again be deducted under any other provision thereof. See also section 7852(c), relating to the taking into account, both in computing a tax under Subtitle A of the Internal Revenue Code of 1954 and a tax under Chapter 1 or 2 of the Internal Revenue Code of 1939, of the same item of deduction.
26 C.F.R. §1.161-1