A double-yolk egg occurs when a chicken's eggs get backed up in their oviduct system, causing two yolks to be encased in a shell. The occurrence is very rare, happening in about one out of 1,000 eggs, which has lead some people to believe various superstitions are attached to breaking open an egg that has two yolks in it.
One superstition attached to opening a two-yolk egg is the belief that someone who does this is pregnant or has someone close to them that is pregnant. Another belief is that someone who does this will be getting married soon. A Wicca belief is that whenever someone does this, they will have lots of good fortune coming to them. Norse mythology, however, has a very negative connotation attached to double-yolk eggs. The belief is that when one is cracked a death will occur in the family soon afterward.
There are actually many kitchen-related superstitions, with plenty having to do with the egg shells left over from cracking open eggs. Farmers used to spread crushed eggs shells onto their fields in the hope it would bring them a plentiful yield of crops. This could be why eggs have their origin in being associated with bountifulness, good fortune and fertility. It is also believed that people should always crush their eggshells after they use them; in earlier times, it was believed that a witch would otherwise gather them up and use them for spells to create terrible storms on the sea.