There are four complete lists of the twelve apostles in the New Testament (I am including the Acts list which does not list Judas Iscariot though he is mentioned in the chapter). They are found in Matthew 10:2-4, Mark 3:16-19, Luke 6:13-16 and Acts 1:13. Ten of the twelve apostles are the same in all four lists. Two of the lists name Simon the Canaanite and two mention Simon Zelotes. Certainly, this is one and the same person. That leaves Thaddaeus in Matthew and Mark and Judas the brother of James in Luke and Acts. By elimination they must be one and the same.
There seem to be a difference in the names of the twelve disciples found in Mark 3:16-19 and Luke 6:13-16. The only solution for this contradiction is to ASSUME that Thaddaeus is Judas. Do you have any proof of this?