I would definitely not do it purposefully either, its just asking for trouble. Most breeders 'line breed' ie double up on a particular cat in a pedigree, to cement a particular good point/type in the offspring, but even that should be done with care as you have to be completely sure that the line you are doubling up on is free from any genetic anomalies - which is almost an impossible thing to ascertain as you could have recessive nasties hidden being passed unseen from generation to generation just waiting to be doubled up on.

Sorry 'Soap Box Alert'

If close family members are deliberately bred the chances of getting problems is enhanced greatly. I have seen breeders do it and proclaim their lines are fine because they had no birth defects or dead kittens, but there is no way of telling at that point, even at 14 weeks when they leave home, what inherent weaknesses you have bred into those kittens, cancers, heart problems, kidney or liver problems, all these things could come out later in life and cause not only suffering for the cat, but heartache for their family. Its illegal for people to marry close relatives for a reason.