Q. 1. What does it mean that reincarnation is a heresy?
A. 1. Reincarnation is a heresy because its teaching involves the belief that certain persons died and came back at a later time as another person. For example, John the Baptist who is mentioned in the New Testament is believed by some to be a reincarnation of the Old Testament prophet Elijah.
Then there are groups such as the "Army of Mary" that had its followers believe that the Virgin Mary, the Mother of God, "reincarnated" and "dwelled" in the body of the founder of their group who was the recipient of alleged private revelations. The Catholic Conference of Bishops condemned this group.
We die once and we are judged once. No one return to be baptised twice, nor to be saved a second time. Christ died for each and everyone of us once only.