From the blog dedicated to human oddities, J. Tithonus Pednaud presents: The Human Marvel:
On occasion, an embryo will begin to split but will fail to complete the process. A dominant twin will begin to consume the other and manifest it as an extra limb, an entire dangling torso or even as a second head. These twins are very much alive and living off the blood of their hosts like a parasite …
Ah yes, ‘the pregnant man’ or - the fetus in fetu case in India - actually not all that uncommon.
I initiailly mentioned that form of parasitic twin in this little teaser, but the animation was graphic and really, not tied into what the human marvels website is all about.
By the way, the most famous living person born with a parasitic twin is Andy Garcia - his brother was attached to his shoulder at birth.