When the survivor is entitled to receive a monthly allowance as a special death benefit provided by Article 1 (commencing with Section 21490), Article 2 (commencing with Section 21530), or Article 5 (commencing with Section 21620) and at the same time is entitled to receive a 1959 survivor allowance, the 1959 survivor allowance payable in any month shall be reduced by the amount of the other allowance or benefit.