[EamonnMR]

Not unheard of, just rare: https://en.wikipedia.org/wiki/Human_chimera

And worth noting that if all goes well, you'd never know you were chimeric.

[thaumasiotes]

A human chimera would not have some cells with a genome from one parent and other cells with a separate genome from the other parent. That's not the way human reproduction or genetics works.

[Brian_K_White]

Unheard of. The wikipedia article is about individual flukes not species norms. This was covered in the article.