The fact that one other person messed up 7 years ago the same way you did, gives you no rights. You would have a valid complaint if you could show that this was due to discrimination because of your age, race, sex, national origin, or disability. However, one other incident 7 ears ago hardly shows a pattern enough to show discrimination.
I also agree that it would not hurt to see the union about this. There may have been some breach of the union contract involved. However, I would not count on help from the union under the facts you gave.