5 C.F.R. § 837.802 – Benefits under another retirement system for Federal employees based on the most recent separation.

(a) Generally. An annuitant who has performed reemployment service after the commencing date of annuity under the provisions of another retirement system, and who is entitled to an annuity benefit from the other retirement system during a period in which he or she is also entitled to an annuity benefit under CSRS or FERS, may receive both benefits simultaneously, or for the same period, except that the annuitant may not receive both benefits simultaneously, or for the same period, if—
1. (1) The provisions of law or regulation governing the other retirement system do not permit the annuitant to receive both benefits simultaneously, or for the same period of time; or
2. (2) Entitlement to the annuity from the other retirement system is based on service credited in the computation of the CSRS or FERS annuity, or service credited in the computation of the annuity from the other retirement system was used in the computation of the CSRS or FERS annuity.
(b) Election of alternative benefits.
1. (1) Where simultaneous receipt of, or entitlement to, both annuities is barred under the provisions of paragraph (a)(1) of this section, the annuitant must elect to receive either the annuity under the other retirement system, or the CSRS annuity.
2. (2) Where the annuitant, under the provisions of paragraph (b)(1) of this section, elects to receive annuity from the other retirement system in lieu of the CSRS or FERS annuity, the CSRS or FERS annuity terminates as of the commencing date of the other annuity, and any overpayment of CSRS annuity will be offset from the other annuity and paid to OPM.
(c) Recomputation. Where simultaneous receipt of annuities from more than one retirement system is barred by paragraph (a)(2), but not by paragraph (a)(1), of this section, the CSRS or FERS annuity may be recomputed to exclude credit for service credited in determining entitlement to, or the amount of, the annuity from the other retirement system, effective as of the commencing date of the annuity from the other retirement system for Federal employees, and the recomputed CSRS or FERS annuity may be paid simultaneous with, or for the same period as, the annuity from the other retirement system for Federal employees.
(d) Forfeiture. Where an annuitant's coverage as an employee under another retirement system, whether by election or by operation of law or regulation, results in forfeiture of annuity rights under CSRS or FERS, the CSRS or FERS annuity will terminate as of the effective date of coverage.
(e) Survivors. The rules detailed in this section in regard to dual entitlement to annuity benefits under CSRS or FERS and another retirement system also apply to dual entitlement to survivor benefits under CSRS or FERS and another retirement system, unless the particular circumstance is otherwise governed by specific provision of statute or regulation.
(f) Agency responsibilities. The agency responsible for administering another retirement system must—
1. (1) Promptly notify OPM of an election of coverage under that retirement system by a reemployed CSRS or FERS annuitant, or the coverage of a reemployed CSRS annuitant under that retirement system by election or operation of law or regulation, when such coverage affects the annuitant's entitlement to CSRS annuity;
2. (2) Promptly notify OPM when a reemployed annuitant separates with entitlement to an annuity under the other retirement system that cannot, under the provisions of paragraph (a) of this section, be paid simultaneous with, or during the same period as, the CSRS annuity; and
3. (3) Reimburse OPM for overpayments of annuity resulting from a failure to comply with paragraphs (b) (1) and (2) of this section.