It's quite common. Not just with Muslims, but with a wide variety of third-world groups. And it was pretty common in the first world too, until pretty recently, like late 19th century

I've read that *most* Americans have cousin+cousin inbreeding as a frequent staple of their family tree a couple hops up. I know I do, and am already planning to rickroll the peanut-gallery-type shenanigans that I hope follows in this thread, cause I certainly have no corner on the downs syndrome market.