During in vitro fertilisation, many thousands of sperm are placed around the egg, many more than are present during natural fertilisation. There is therefore the possibility that two sperm will enter the egg at exactly the same time. This probably happens in some natural cycles, but not as commonly.
Eggs which are fertilised with two sperm are highly unlikely to develop normally. There is even the chance that they will develop into what is called a molar pregnancy, where the placenta grows as a series of cysts, and there is either no fetus or the fetus dies. Molar pregnancies may sometimes go on to behave like a cancer, so eggs that can be shown to have fertilised with two sperm (called tripronuclear) are not transferred into the uterus during IVF treatment.