Legal technicalities aside ... I still suspect they thought the first one had expired! 
They may have,
but surely the licenced person performing the NSW marriage should have made enquiries .... That was certainly a function that the 19thC clergy carried out diligently ... so many who sought permission to marry were denied the opportunity after the Clergy made enquiries .... surely that is still part of the licencing functions for both clergy and civil celebrants even today?
Although .... perhaps .... the NSW one was to make sure .... I'm thinking of say drivers licence ... as I recall, these were not necessarily recognised in each state in the 1960's ...
Were they shown as bachelor/spinster on the NSW one ... Did they get their parent's names right ! Oh Deb, I am absolutely amazed ... It must be a good talking point within the cousins with keen family history interests.
Mind you, there was much in the NSW papers even in 1952 re Anthony Eden's re-marriage
http://trove.nla.gov.au/ndp/del/article/29439321 SMH 20 August 1952
Or this one ...
http://trove.nla.gov.au/ndp/del/article/18054189 19 December 1947 SMH discussing the UK Lord Chancellor's pronouncements in the House of Lords re a divorce matter. (Australia was in 1947 still a dominion of Britain)
Cheers, JM
