It is harmful. We live in an Amish and Mennonite area with a lot of 2nd or 3rd cousin marriages (over generations)and marriages among people who are “double” cousins (both sides of the family). The local doctors advise against it. A local family lost a baby last because the couple were second cousins whose families had intermarried for years.
I used to drive for Trailways and had a lot of Amish passengers. There was a very high percentage of children with defects.