In rare cases, two sperm (from one man) can enter one egg at the same time, resulting in 3 sets of genes rather than two. This is called triploidy. Most cases of triploidy are not carried to term. On the rare occasion of a live triploidy birth, the child often has serious birth defects. After doing some research, I have not found any cases of triploidy in which one egg was fertilized by two different men. However, it is possible for a woman to have fraternal twins with different fathers. A woman typically drops only one egg per menstrual cycle from her ovaries, but once in a while two eggs will drop. If a woman has unprotected sex with two men within about 5 days of each other (sperm can survive inside a woman for about 5 days), and if she happens to drop two eggs at that time, each egg could be fertilized by a different guy.