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READ THIS FIRST - Private Parking Charges Forum guideraging that I would have to pay another for the same parking on the same day .
It would be good to understand what you mean by "the same parking", are you saying your vehicle did not leave the car park between the times of the two parking charge notices?
If you have missed the appeals window, you will start receiving letters from debt collectors. These letters are annoying, but can be ignored, as debt collectors are powerless. The next step after that, if SIP choose to, is to sue you in the County Court (Small Claims), if you receive a Letter of Claim from solicitors acting on SIP's behalf, or SIP themselves, come back here for advice.