I have no idea, honestly. If it was me, I would write them a letter claiming that they made a mistake, and I would attach a copy of my parking fee receipt. I would send it out as a registered mail, and I would keep a copy of my letter and my oriiginal receipt just in case.
Result? I don't know. Maybe they will write it off. Or maybe they want to sue you for not paying the fine. Who knows. You can only determine what to do once you receive their reply, if they reply at all. At least, if they want to sue you, usually you will receive a notice of their intention. Or maybe they just go ahead and sue. If you want to have some fun, maybe practise going to court and see what would happen, you can defend yourself in court. Why not. It's probably worth the fun.