Thanks chaps, I hadn't thought of twins. There is a time on both certificates, but it's difficult to make out. One of them looks like "10h, a, m" whilst the other looks like "10h, 15m by m". I think maybe twins is the answer, and that the one that has "15m" noted on it is possibly 15 minutes after the first - if that's possible.
More than possible in my opinion. The reason is to allow for inheritance disputes if primogeniture is invoked - not likely in the lives of ordinary people. Some registrars entered times for many, even most, births, so it doesn't always indicate a twin.
