You can't appeal to Victorian morality; Queen Victoria married her first cousin. You can't appeal to the Bible; in the Bible, God commands marriages between first cousins. Instead, advocates of laws against cousin marriage appeal to science. To let cousins marry, they argue, is to play Russian roulette with genetics.. Many genetic diseases are caused by recessive genes.
http://www.slate.com/articles/news_and_politics/frame_game/2002/04/the_love_that_dare_not_speak_its_surname.html
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.   Double second cousins would be the children of double first cousins.   Two cousins are removed when they are are a different number of generations from their shared ancestor. When that difference is one generation, they are once removed.
http://www.answers.com/Q/What_is_double_cousin_marriage
First cousins are two children who share a set of grandparents, but have different parents. Double first cousins share both sets of grandparents, but have different parents. Let's state that in a different way: double first cousins happen when two siblings of one family marry two siblings of another family. When you and your brother marry Bob and his sister, your children and their children will be double first cousins. Many states in the USA allow first cousins to marry *; but as far as I can find only one, North Carolina, will allow cousins to marry but will not allow double first cousins to marry.
http://www.everything2.com/title/Double+first+cousin
When identical twins reproduce with siblings the resulting children are more related than half-siblings but less related than full siblings. Children of double first cousins are double second cousins.
http://familypedia.wikia.com/wiki/Double_first_cousin
It occurs when two siblings in one family marry two siblings in another family. An example would be .... Sally and Sue Smith are sisters. Joe and John Doe are brothers. Sally … married Joe...they have a child Rick, Sue marries John and they have a child Mary. Mary and Rick are double first cousins.
http://www.answers.com/Q/What_is_double_cousin_marriage
Double first cousin. 192,101 pages on this wiki. 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.
http://familypedia.wikia.com/wiki/Double_first_cousin
Now a study by the National Society of Genetic Counselors says that having a child with your first cousin raises the risk of a significant birth defect from about 3-to-4 percent to about 4-to-7 percent.
http://www.slate.com/articles/news_and_politics/frame_game/2002/04/the_love_that_dare_not_speak_its_surname.html
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.
http://familypedia.wikia.com/wiki/Double_first_cousin
Double cousins are formed when two siblings from one family marry two siblings from another family, and both couples have children. For example, Jane and Elizabeth are sisters.
http://www.answers.com/Q/What_is_double_cousin_marriage
To get the disease, you have to get the bad gene from both parents. The greater the genetic similarity between your parents, the greater your chance of getting two copies of the bad gene. But if that's your reason for banning cousin marriage, you've drilled into a mother lode of problems.
http://www.slate.com/articles/news_and_politics/frame_game/2002/04/the_love_that_dare_not_speak_its_surname.html
what are double cousins?
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.