Why We Have Double Cousins

Here is a photo of My Dad (right front) and his brother Art, (left front) behind Art is his wife, June and behind Dad is her sister, My Mom, Bertha. So that's how you get to be Double Cousins! Mom and Dad had 6 children and Art and June had 8. So lots & lots of Double Cousins! Some of us look more like brother and sister than our own brother and sisters!
In between June and Mom is Art and Dad's younger brother, Vernon. I am guessing this picture was taken in 1943 or 44?

Double first cousins arise when two siblings reproduce with another set of siblings and the resulting children are related to each other through both parents' families. Double first cousins share both sets of grandparents in common and have double the degree of consanguinity than ordinary first cousins. Genetically they are as related as half-siblings. Their coefficient of coancestry is 1/8th or 0.125. While double first cousins have the same coefficient of coancestry (1/8) as half-siblings, they do have higher chances of sharing both alleles (1/16 vs 0) and lower chances of sharing one allele (3/8 vs 1/2) with each other than half-siblings.