If the paperwork indeed has the same number then this is a scandal and there should be an enquiry! The chip company should pick up the tabs for any Xrays.
According to the paperwork, when where both cats chipped and by whom? was it the same vet? When did the cat go missing and where? does this co-incide with the appearance of the cat? The only other thing that springs to mind is that there are 6 labels on each chip and if they were chipped at the same time/place then there is a small possibility of human error and the labels were used on two registration forms - and this is stretching it.
I have been told before that sometimes chips don't always scan first time, that they need to somehow 'energize' . I always triple check for chips now just in case.
I did have a case in Coventry when I was he petsearch contact here - think I even put in on purrs. A lady lost a cat and one was found, it was chipped but the vn read the chip number wrong . The lasdy who lost the cat was adamant it was hers - looks, temperament and even some of the odd behaviour.
Eventually, when the chip was read correctly, it turned out to ba another missing cat a couple of miles away and when peaceful negotiation broke down the cat was 'stolen' by the owners it was registered to. Not quite the same as there was no error in the chip (we can only assume
) but it does show that owners do not necessarily know their cats as well as they think.