One of the sages, Flimo, asked Rabbi Judah the Nasi: If one has two heads, on which should he place tefillin? Rabbi Judah the Nasi answered him angrily, assuming he was being mocked by this unrealistic question, and told Flimo to either leave the place and go into exile or be excommunicated and isolated from people. Later on an incident occurred and a baby was born with two heads. The baby’s father came to Rabbi Judah the Nasi and asked him: I have an eldest son who was born with two heads. How much do I pay to redeem him — the value of one son, which is five sela [approximately 1800 shekel] or of two, which is 10 sela [3600 shekel]? He was answered: The payment must be for two sons, 10 sela. The students asked: Why did the rabbi require he pay a double redemption price for the infant with two heads? An eldest son born with a defect which will kill him is exempt from the redemption of the first-born, and being born with two heads is a fatal defect. Answer: Since the laws of the Torah about the redemption of the first-born specify “You shall take five shekels for each head” (Nu,bers 3:47), it means that the laws of the redemption of the first-born go by heads, and since this first-born has two heads, the father must pay double to redeem him.
(Babylonian Talmud, Tractate Menachot 37a-b)