How closely related are double first cousins?
Double first cousins are not related in the same way as
regular first cousins. In fact, they are considered to be twice as related. They share all four of their grandparents in common and share 100% of their ancestors. This is twice as much as the common 50% that most cousins share.
...
Do double first cousins have the same DNA?
Double cousins share twice the amount of DNA as typical first cousins. The average DNA shared by two first cousins is around 12.5%. However, double first cousins share an average of 25% of their DNA.
This is the same approximate figure as half-siblings. Therefore, double first cousins share around the same amount of DNA as half-siblings.