I'm not entirely sure how useful this will be, particularly as it mainly concerns modern people, but I thought there might be someone out there that would find this beneficial so may as well share.
My parents are unmarried and I discovered that I seem to be on the GRO birth index twice - or, on Ancestry's version of the index, at least. Both entries are identical, the only differences are one has mine and my dad's surname (Dowding) and the other had my mum's surname listed as my surname. Which was odd as both had my mum's surname under "Mother's Maiden Name".
My initial assumption was that my parents weren't telling me something, lol, but I knew that my birth certificate did indeed list me as a Dowding. After some searching around, I found that this same thing seems to be true for everyone in the family with unmarried parents, whilst it doesn't seem to apply to anyone who had married parents (in which case, there's just the one entry).
There was one exception - one of my dad's sisters changed her surname, then got married (so another change of surname) and then had a child. The child had two birth entries - both had "Mother's Maiden Name" listed as the mum's first name change instead of Dowding, one had the child's surname listed as both her parents' married surname and the other had the child's surname listed as Dowding. Bit odd.
I don't know the technicalities or the date range. All the people that this seems to be the case for were cousins born in the 1990s/2000s. My grandfather did have two half-brothers born in the 1950s to unmarried parents and this doesn't seem to apply to them, however that could be because their mother's maiden surnames both say Dowding when Dowding was in fact the father's surname.
