I had come across a few of the double entries, and all were by licence. My initial reasoning was that the marriage took place in the bride's Parish but the service was conducted by groom's encumbent.
BUT... on a pro-forma register where blanks were to be filled in, there was a big clue to the answer. The pro-forma had - were married in this Church - and the groom's Parish register had the 'in this Church' crossed through, so my theory is that because there were no Banns published, and the marriage was in a different Parish, the groom's encumbent announced the marriage to his congregation and entered the details in his register. No wagging tongues in his Parish!!
Colin