Marriages between close relatives have been quite common in a number of cultures. The inbreeding in several European noble families is well-known. Several sources report that some Polynesian tribes also applied a "nobility must marry nobility" tradition.

Livestock breeders frequently use inbreeding to improve their line, but of course with the option of "culling" any seriously "defective" offspring. (They're all going to be hamburger eventually anyway?). This compensation for the occasional weaknesses is frowned upon in most human cultures, but there's good evidence it was practiced to some extent by some.

In the "wild frontiers" in the US, cousin/cousin marriages were more common than probably is widely known, since often there were extremely limited numbers of "marriageable partners" available.

While the "Incest Destroyer" name for the App reported gets one's attention, the more interesting part for those with genealogical interests likely will be the depth and completeness claimed for the Icelandic database. A question is whether there are other "isolated" cultures where similar information is suspected.

John