Cheating, or an Early Mingling of the Blood?

New York Times: "Last month, when the champion American cyclist Tyler Hamilton was accused of blood doping, or transfusing himself with another person's blood to increase his oxygen-carrying red cells, he offered a surprising defense: the small amount of different blood found mixed in with his own must have come from a 'vanishing twin.' In other words, his scientific expert argued, Mr. Hamilton had a twin that died in utero but, before dying, contributed some blood cells to him during fetal life. And those cells remained in his body, producing blood that matched the dead twin and not Mr. Hamilton."