I have been trying to get my double second cousins to do DNA testing but they will not, but I would love to see the results.They are descended from both my mother's mother's family and my mothers' father's family, with my maternal grandmother's 1st cousin marrying my maternal grandfather's first cousin in 1920. 

I also have a situation where my maternal great grandparents were first cousins, and then one of their sons married the daughter of my great grandmother's brother, making them double first cousins.  Ancestry's family tree cannot cope with that and so I have had to enter them twice. 

But we are commonly related several times over to our different families. When another of my 2nd cousins tested a few years back, I also found I was related to his wife. I have the situation where some members of my father's family are distantly related to my mother's family - not surprising as they came from the same geographical area. So endogamy will be the main reason for the result. 

Jill Whitehead, Surrey, UK