It's true this member might not repeat his mistake anymore and you can still forgive someone for their second offense, but it looks unfair for old member who get banned because of one old plagiarized post.
That depends on moderators! If they feel like they were much stricter back in the old days, and now they want to give chances for the first or 2nd offense, I do not see anything wrong here. The old members who were banned for their first offense should adjust themself and take it with a light heart. If they feel, "I was banned, now they should be banned too," there is no double standard.
I want to say that rules change over time. Sometimes, the same rules apply in different ways. So, old members should let the moderators decide what they should do. All you need is just report and let them do what they want.