Who was the disciple Judas who was not Judas Iscariot?

Judas is either the brother or son of James, depending on whether you are reading the KJV or the RSV. Assuming this James to be James Alphaeus, we either have three Alphaeus brothers, or two brothers and a son and nephew.
The reason that there are thirteen disciples rather than twelve, is that Luke 6:16 identifies Judas the son of James as a disciple, and eliminates Thaddaeus/Lebbaeus from the list.
Judas the son of James is not mentioned by the other Gospel writers, and Thaddaeus/Lebbaeus is not mentioned in Luke.

Answer #2

Judas, the son of James, was mentioned in Luke 6:16. In the Gospel of Mark he is called Thaddaeus where this was his Greek name and Judas was his Hebrew name.

People in that culture often had multiple names so for example Mark the apostle was also called John Mark.