Did you mean that if someone plagiarise today or two months ago, they are deserved with perma bans, whilst someone whom did it years ago should be considered as innocent. Plagiarism comes from copy & paste. Which forces lead to copy & paste (lack of time, stealing other ideas, lazy to compose own posts, financial benefits by stealing other ideas)?

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Another point is there are actually second chances for good users, even they plagiarised in the past. Admin, actually made perma-banned users second chances if they have good net-effects. If not, admin say no with any second chance.
If you, yourself, think that you are not quality enough to take such a second chance given by admin. It's your second fault (after the first one - past plagiarism), not admin's.