There seem to be a difference in the names of the twelve disciples found in Mark 3:16-19 [16] And Simon he surnamed Peter; [17] And James the son of Zebedee, and John the brother of James; and he surnamed them Boanerges, which is, The sons of thunder: [18] And Andrew, and Philip, and Bartholomew, and Matthew, and Thomas, and James the son of Alphaeus, and Thaddaeus, and Simon the Canaanite, [19] And Judas Iscariot, which also betrayed him: and they went into an house.
See All... and Luke 6:13-16 [13] And when it was day, he called unto him his disciples: and of them he chose twelve, whom also he named apostles; [14] Simon, (whom he also named Peter,) and Andrew his brother, James and John, Philip and Bartholomew, [15] Matthew and Thomas, James the son of Alphaeus, and Simon called Zelotes, [16] And Judas the brother of James, and Judas Iscariot, which also was the traitor.
See All.... The only solution for this contradiction is to ASSUME that Thaddaeus is Judas. Do you have any proof of this?

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 [2] Now the names of the twelve apostles are these; The first, Simon, who is called Peter, and Andrew his brother; James the son of Zebedee, and John his brother; [3] Philip, and Bartholomew; Thomas, and Matthew the publican; James the son of Alphaeus, and Lebbaeus, whose surname was Thaddaeus; [4] Simon the Canaanite, and Judas Iscariot, who also betrayed him.
See All..., Mark 3:16-19 [16] And Simon he surnamed Peter; [17] And James the son of Zebedee, and John the brother of James; and he surnamed them Boanerges, which is, The sons of thunder: [18] And Andrew, and Philip, and Bartholomew, and Matthew, and Thomas, and James the son of Alphaeus, and Thaddaeus, and Simon the Canaanite, [19] And Judas Iscariot, which also betrayed him: and they went into an house.
See All..., Luke 6:13-16 [13] And when it was day, he called unto him his disciples: and of them he chose twelve, whom also he named apostles; [14] Simon, (whom he also named Peter,) and Andrew his brother, James and John, Philip and Bartholomew, [15] Matthew and Thomas, James the son of Alphaeus, and Simon called Zelotes, [16] And Judas the brother of James, and Judas Iscariot, which also was the traitor.
See All... and Acts 1:13And when they were come in, they went up into an upper room, where abode both Peter, and James, and John, and Andrew, Philip, and Thomas, Bartholomew, and Matthew, James the son of Alphaeus, and Simon Zelotes, and Judas the brother of James.
See All.... 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.

The only other logical possibility would be that these are indeed two men and one replaced the other during the ministry of Christ. However, this is not possible because of a couple of things. First, there is no hint of any such switch. But second (and conclusive) is the fact that Mark 3:13And he goeth up into a mountain, and calleth unto him whom he would: and they came unto him.
See All...- 9 and Luke 6:12-16 [12] And it came to pass in those days, that he went out into a mountain to pray, and continued all night in prayer to God. [13] And when it was day, he called unto him his disciples: and of them he chose twelve, whom also he named apostles; [14] Simon, (whom he also named Peter,) and Andrew his brother, James and John, Philip and Bartholomew, [15] Matthew and Thomas, James the son of Alphaeus, and Simon called Zelotes, [16] And Judas the brother of James, and Judas Iscariot, which also was the traitor.
See All... are parallel passages recording the same event. In both, Jesus went up to a mountain and then returned. In both, He ordained twelve men to be His apostles. This could only refer to one event and time. In Mark, one on the list is Thaddaeus. In Luke, the parallel name on the list is Judas the brother of James. They must be one and the same.