The example you're looking at right now is calcite, and when you're seeing double, it's because light with one polarization is getting bent at a different rate than light with the other polarization.
您现在看到的例子是方解石,当您看到双重时,这是因为具有一种偏振的光与具有另一种偏振的光以不同的速率弯曲。