Polyspermy
In biology, polyspermy describes an egg that has been fertilized by more than one sperm.  Diploid organisms normally contain two copies of each chromosome, one from each parent.  The cell resulting from polyspermy, on the other hand, contains three or more copies of each chromosome -- one from the egg and one each from multiple sperm.  Usually, the result is an inviable zygote. This may occur because sperm have become too efficient at reaching and fertilizing eggs due to the selective pressures of sperm competition. This is often deleterious to the female, in other words the male-male competition among sperm spills over to create sexual conflict.

# Evolved defenses against polyspermy
The eggs of sexually reproducing organisms are adapted to avoid this situation. The defenses are particularly well characterized in the sea urchin, which responds to the acceptance of one sperm by inhibiting the successful penetration of the egg by subsequent sperm.  Similar defenses exist in other eukaryotes.
The prevention of polyspermy in sea urchins depends on a change in the electrical charge across the surface of the egg, which is caused by the fusion of the first sperm with the egg.  Unfertilized sea urchin eggs have a negative charge inside, but the charge becomes positive upon fertilization.  When sea urchin sperm encounter an egg with a positive charge, sperm-egg fusion is blocked.  Thus, after the first sperm contacts the egg and causes the change, subsequent sperm are prevented from fusing.  This "electrical polyspermy block" is thought to result because a positively charged molecule in the sperm surface membrane is repelled by the positive charge at the egg surface.
Electrical polyspermy blocks operate in many animal species, including frogs, clams, and marine worms, but not in the several mammals that have been studied (hamster, rabbit, mouse).   In species without an electrical block, polyspermy is usually prevented by secretion of materials that establish a mechanical barrier to polyspermy.  Animals such as a sea urchins have a two-step polyspermy prevention strategy, with the fast, but transient, electrical block superseded after the first minute or so by a more slowly developing permanent mechanical block.  It is thought that electrical blocks evolved in those species where a very fast block to polyspermy is needed, due to the presence of many sperm arriving simultaneously at the egg surface, as occurs in animals such as sea urchins.  In sea urchins, fertilization occurs externally in the ocean, such that hundreds of sperm can encounter the egg within several seconds.
In mammals, in which fertilization occurs internally, fewer sperm reach the fertilization site in the oviduct. This may be the result of the female genital tract being adapted to minimize the number of sperm reaching the egg.  Nevertheless, polyspermy preventing mechanisms are essential in mammals; a secretion reaction, the "cortical reaction" modifies the extracellular coat of the egg (the zona pellucida), and additional mechanisms that are not well understood modify the egg's plasma membrane. 
Female defenses select for ever more aggressive male sperm however, leading to an evolutionary arms race. On the one hand, polyspermy creates inviable zygotes and lowers female fitness, but on the other, defenses may prevent fertilization altogether. This leads to a delicate compromise between the two, and has been suggested as one possible cause for the relatively high infertility rates seen in mammalian species.