I don't see why not. YOu only have to look at things like 2 headed sheep to see that we are all freaks of nature one way or another, and just because we are human doesn't make us any different from animals, plants etc. The human would have to have not just functioning reproductive organs, but all the inner bits to make and store sperm as well as house and grow a baby. Many hermaphrodites have versions of male and femail genitals, but may have female insides etc. In theory, if they had all these necessary parts, they could impregnate themselves.