Recent concepts regarding the origin of FIF suggest that it is part of a spectrum of monozygotic twinning gone awry, ranging from conjoined twins at one end to fetaform teratomas at the other.
In other words, whether you once had a conjoined twin, or a parasitic twin, or a fetus in fetu, or a teratoma or even a demoid cyst, then the only conclusion that these doctors can come to is that this is your identical twin, enclosed within your own body.
This man has his twin inside him:
And this baby has a parasitic twin
But all ended well... he is now a womb twin survivor because his twin has been removed.