I suggest you look at the Parish registers for his baptism, it looks to me that John Collinge may have been the illegitaimate son of sarah but the natural son of Peter, if they married subsequent to his birth registartion they may not have known they could go back and reregister the child as legitamate, well it's not regiegstering really but the record is changed. By the time John married he was an adult and he knew he had been brought up as Duffy and he knew he was his fathers son and so he was entitled to put peter as his father. Also , it is probable that the boys were twins and the eldest born was always going to be called John after a grandparent, but the eldest is the boy that died at 30 mins and was buried as John, but then when the registartion was done, and you may need to check the date of registartion against the date of birth, probably they decided to use the name for the younger twin. If both twins had survived to registartion of the birh there would be a time on the certificate but it is unlikely the boy that died had his birth registered though these days I think both would have been done. It sounds feasible in my head, just have a think about it and tell me if any of that works.