It's called line-breeding, and it's not quite as problematic as inbreeding (breeding sibling to sibling), but it's not something you want to do several times in a row. The danger with incestuous breeding is that any genetic flaws (as well as desireable mutations) will be fixed and expressed. This makes genetic flaws pile up, and increases the risk of low fertility and defects. In general, reptiles appear to be somewhat sturdier when it comes to holding up under incestuous breeding than mammals are, but eventually they too will fall prey to a pile of recessive defects that have been made visible through generations of in/line breeding.