Private Ambulance Service Charges

You have essentially two options to get this before a court, since only a court can resolve this for you:
1) Refuse to pay; ignore their attempts to collect; IF they sue you, then prove in court that you paid and the court will order them to correct their ledgers and stop their collections efforts. This approach means you will likely have to put up with their harassment for a time, even if they ultimately choose to not sue or pursue legal action; may have a bad debt reported to credit rating agencies; and give the initiative to them--you won't know if or when they drop the matter (rather than suing). The advantage is that you only have to go to court if they do try to sue you for the money, and the type of defense you would mount if sued is a straightforward one--you could very reasonably represent yourself ("pro se") and not have to pay for an attorney.
2) You could go "on the offenseive," so to speak, and bring a lawsuit against them seeking a "declaratory judgment" or court determination that you have already paid and do not owe them anything, as well as a court order that they stop pursuing you for the money and do not report a bad debt to credit agencies (or if already reported, that they correct the report and provide proof that they did so). The advantage is that this puts the initiative in your hands and lets you *know* that the matter is resolved. The disadvantages are that you now definitely have to go to court, and that because asking for a declaratory judgment and a court order is more procedurally complex than bringing or defending a case for money only, you would most likely need to hire a lawyer and incur that cost.