In a case where PCNs dated after the first one, and dated before the date of receipt of the first PCN, but only received after receipt of the first PCN, these can be argued on the basis of not knowing an offence had been committed until the first PCN arrive in the post.
One would normally submit a single representation with a list of the PCNs, and a request to cancel, pointing out the first PCN (give number) has been paid.
Of course the council may not give way, but it is well worth trying. Many councils do give way, so don't be put off. Would this argument win at adjudication ? It might and has done in the past, but its not a slam-dunk argument, I'm afraid.