I have an example of pedigree collapse in my direct line, within my Jewish portion of my tree - a pair of first cousins who married (my 4th great-grandparents on my paternal grandmother's side); their mothers were sisters. It can get a little confusing when you realise the husband chose to take his mother's surname rather than his father's, and they were in the midst of anglicising their names as well... The sisters were born in Holland and their families seemed to have been immigrating into London at the time.
Ironically, on the same side of the family, there are a bunch of extended family members who intermarried. Perhaps first, maybe 2nd cousins and an example of what seems of a niece/great niece marrying what looks to be a great uncle (this pair is not related by blood, but rather by marriage, but it's also a bit strange!). This particular branch of the family were most definitely non-conformists (Baptists I believe), so this could have been encouraged in some ways, but it was also a massive family.