How rare is 2 full moons?

I'm curious, how uncommon is it to experience two full moons within a given time period? Is this a rare occurrence in the lunar cycle, or is it something that happens relatively frequently? I've heard of blue moons and other lunar phenomena, but how does the occurrence of two full moons compare? Is there a scientific explanation behind why this might happen, and does it have any significance in terms of astronomy or astrology?