Jesus had an unknown identical twin that impersonated Jesus after stealing Jesus' body from the tomb. Moreover, the parents of Jesus and his twin brother never knew that the real baby (the twin) got accidentally switched with another baby. Thus, Jesus' twin brother grew up independently of him.
Apart from the fact that this attribution of Jesus as having a twin is so far removed from the original life of Jesus as to be legendary, and the notion of twin in the Book of Thomas the Contender may be 'spiritual,' or 'relational,' and not biological, let's assume that Jesus really had a twin.
1) It has weak explanatory scope because it has to conjoin itself to a hallucination hypothesis in order to account for the post-mortem appearances of Jesus.
2) It has weak explanatory power. It cannot explain the supernatural aspects of the appearances, namely, they had a numinous aspect, they weren’t earthly (every New Testament Scholar agrees on the properties of these appearances). Moreover, the wounds of the crucifixion are also an aspect that the identical twin hypothesis cannot explain. These aspects are independently attested and part of the evidence to be explained, and are accepted by most historians. In addition, it cannot adequately explain the origin of the disciples belief in a resurrected Jesus. It seems wild to suppose that Jesus' disciples wouldn't notice a difference in Jesus' twin because Jesus was radically unique in terms of his teachings, character, and actions
3) It is very implausible. An accidental switch of babies is very implausible because Jesus was either born in solitude in a stable or else, in the stable under the one roof of a Palestinian home. In the first case, there were no other babies around, and in the second case, Mary and Joseph were under the care of the relatives who owned the home. Judaism had no notion of a crucified Messiah, or a resurrected individual prior to the end of the world, and Jesus’ crucifixion meant he was cursed. But, on the twin theory, Jesus’ twin goes to Jerusalem to satisfy his curiosity upon hearing about his twin brother, he arrives just as Jesus is being crucified, and being an unethical guy he then conceives the idea of hoaxing Jesus’ resurrection. However, you would have to be a moron to want to go around impersonating someone who the Jews had just crucified for being a heretic, and for who the Romans had crucified for treason. The main point however, is that is would never occur to Jesus’ twin to hoax a resurrection.
In addition, the kind of tomb Jesus was buried in would have been sealed with a very heavy slab of stone that one man alone couldn’t move. But, if Jesus' twin couldn't move the slab, then he couldn't steal the body which means that the tomb wouldn't have been empty for any natural reason. And if there was no empty tomb, then a belief in a Resurrected Messiah would never have arisen.
4) It is outrageously ad hoc. There is absolutely no evidence to think that Jesus had a twin brother, that Mary wasn’t Jesus mother, that Jesus’ twin grew up apart from him, that Jesus’ twin was unethical, that Jesus’ brother learned he had a twin, that he came to Jerusalem just as Jesus was being crucified, that he wanted to hoax a resurrection.
5) It is disconfirmed by accepted beliefs. It is universally accepted that Jesus was the biological son of Mary and had no identical twin.
TAKEN FROM THE DEBATE WLC HAD WITH GREG CAVIN