"The purpose of the subsequent injuries legislation is to encourage the employment of handicapped persons by providing for 'additional compensation to be paid from a fund (rather than by the employer or his workmen's compensation insurance carrier) to the already handicapped worker who sustains an industrial injury which causes increased permanent disability.' " (Subsequent Injuries Fund v. Industrial Acc. Com. (1963) 217 Cal. App. 2d 322, 327 31 Cal. Rptr. 508.) To prevent a double recovery of benefits, the California Subsequent Injuries Fund is entitled to a credit for any other disability benefits a worker receives for a preexisting disability, with some exceptions not pertinent here. (Subsequent Injuries Fund v. Industrial Acc. Com., supra, 217 Cal. App. 2d at pp. 329-330, 332.) Thus, the additional compensation paid by SIF is "reduced to the extent of any monetary payments received by the employee, from any source whatsoever, for or on account of such preexisting disability." ( 4753.)