Section 18(3)(a) in The M.P. Civil Services (Pension) Rules, 1976
(a)A Government servant who opts for clause (b) of sub-rule (1) shall be required to refund the pension or gratuity received in respect of his earlier military service, in monthly instalments not exceeding thirty-six in number, the first instalment beginning from the month following the month in which he exercised the option.