Just submit reps asap, on the lines that you are a complete stranger to Preston and managed to get lost whilst trying to <insert reason for being there>. Just say that you accept you passed two separate signs, but would ask for the second PCN to be cancelled, as it is a disproportionate penalty for what is, essentially, a single contravenention, namely driving along the whole length of Corporation Street during late evening.
If they decide you're over the limit for submitting reps, they'll tell you.
