One of my g grandmothers was illegitimate. On her birth and marriage certificates there was a father's name, but we suspected that it was fictitious (especially since the father was deceased on the marriage certificate.)
The father was a fishmonger, which made me think: If I were inventing a father for my child, I would have chosen something a bit more upscale. I started searching, and discovered that the father did exist--but he was married to another woman at the time my g grandmother and her sister were born! And he was still married to the other woman when he died at the age of 70. He and his wife had a number of children, and two of them--born 10 years after the births of my g grandmother and her sister--were given the same names as my g grandmother and her sister! (They were quite unusual names.)
