In John we find one of the 12 apostles is referred to as “the twin”:

Joh 11:16 Then Thomas, who is called the Twin, said to his fellow disciples, “Let us also go, that we may die with Him.”
Joh 20:24 Now Thomas, called the Twin, one of the twelve, was not with them when Jesus came.
Joh 21:2 Simon Peter, Thomas called the Twin, Nathanael of Cana in Galilee, the sons of Zebedee, and two others of His disciples were together.

Although the Bible does not identify of other individual to whom Thomas was a twin, it is safe to assume that Thomas in fact was a twin. People called him the “twin” probably because he had an identical twin brother.

The Gnostics looked at this word “twin” and some variant gnostic sects began to identify Thomas as the twin of Jesus. In the gnostic text, the Gospel of Thomas, the author claims to be Didymos Judas Thomas. In the Book of Thomas, it is even more explicit:

The savior said, “Brother Thomas while you have time in the world, listen to me, and I will reveal to you the things you have pondered in your mind.

“Now, since it has been said that you are my twin and true companion, examine yourself… So then, you, my brother Thomas, have beheld what is obscure to men, that is, what they ignorantly stumble against.”

This has perplexed scholars: How could early Gnostics believe that Jesus was divine yet have a twin brother who was not? Perhaps Bart Ehrman gives the best explanation: It was not uncommon in Greek mythology for a divine son to have a human twin brother. After all Hercules had a mortal twin brother, Iphicles. Leda was said to have born four children at once, two moral and two divine. This was a belief that could be held by the common people.