Second-cousin marriages aren't that uncommon, and they're easy to comprehend: two people get married and they share a common ancestor, specifically a great-grandparent and usually a great-grandfather and great-grandmother. That's consanguinity of the third degree.
Today, I found a weird situation where a married couple's marriage certificate mentioned a third-degree consanguinity which I followed-up, but then noticed that they actually shared TWO sets of great-grandparents. In other words, they're double second cousins. (They're also my 5th great-grand aunt and uncle.)
Then I got to the next marriage in their family tree. The bride and groom have the same sets of parents as the previous couple: a brother+sister marrying a sister+brother. Again, that's not THAT uncommon, but then it hit me: you have two siblings each marrying their double second cousin.
It's a "double" double second-cousin marriage!
So, each of these couples' children (my 1st cousins 6x removed) only have 12 great-great grandparents instead of the usual 16.
It gets WEIRDER.
In 1773, Marie-Angélique Grenier (1729-?), my 6th great-grandmother, remarried after Joseph-Gaspard Choret passed away in 1768 to Jean-François Sévigny who was recently widowed from Marie-Anne Croteau (1719-1772).
This sets up another "Greg and Marcia Brady" situation as my last posting. Fifteen months later, Angélique's daughter Geneviève (1756-1824) who was 17 at the time of the re-marriage, marries her 22-year old step-brother Pierre Sévigny (1752-1828). They're my 5th great-grandparents.