The Twins of Cândido Godói
In the town of Cândido Godói in Brazil there is an odd phenomenon. While the surrounding region has a rate of 18 out of every 1000 pregnancies carried to term result in twins which is pretty much the world average, the town itself has a rate much closer to 100 out of 1000. Note that because each of these pregrancies results in two children, the percentage of people who have a twin much higher.
There have been lots of debate about why this is. The locals blamed something in the water and by all accounts were pleased with the publicity from journalists in the 90s when the world caught on to the story. However a darker explanation soon caught on. All of the residents come from German and Polish ancestries who flees during the Second World War. Known to have visited the village while in exile, Josef Mengele the chief physician at Auschwitz and known experimenter of genetics (in an Aryan not very nice to his victims sort of way) was reported to have done experiments with the local population. It sounds like something out of something from American Horror Story: Asylum, but Mengele is on record for experimenting with twins back in Germany.
This explanation caught on and many of the news articles about it during the 90s and 00s suggest this angle. This turned out not to be the reason. Firstly the birth rate of twins in the region can be seen in the baptism records in the region back in the 30s which predates Mengele's exile. Genetic tests of the region show that there has been a high rate of interbreeding and incest, which is common when a small group emigrates together and then fails to integrate with the local community over decades. A single gene has been found in a large percentage of the population which creates a high rate of twin births. I came across this topic when I was doing the incest mathematics for Game of Thrones. In short, try not to marry someone you are closely related to.