Craniopagus Parasiticus.

Say the words to yourself once or twice. Ten syllables. Cranio-pagus Para-siticus. The biological equivalent of finding an extra large and deformed M&M in your packet. Twins, but wrong. A human life, twisted by a matter of womb-inches, to the status of a tumour with a face. Insane ethical questions. The stubby bit has reflexes and a seperate personality- introduce a bottle to its mouth, and it sucks. It remains awake while the host sleeps. Without its own heart or respiratory system it has no hope of independant survival- it's just a head, after all. But is it human? Is it a person? The entity as a whole- two people, or one with a noteworthy addition, like a third nipple or extraneous thumb? Medical intervention is mandatory, as one heart can't support that much matter, but should the approach focus on excision of the alien mass, or prolonging the survival of the entity as a whole? Given time and care, could the extra head be capable of learning? Is it sentient? And if so, would a surgical procedure to remove it qualify as excision or infanticide?
Craniopagus Parasiticus. Two heads aren't always better than one.