Back to the subject..........
I had this problem with a well known engineering company ( now gone ), I paid up front for a Mag overhaul and they insisted on sending me invoices, each time I got one of these I called their accounts people and told them I had paid...... they then looked at the records and agreed I had paid the bill in full.
this went on for about a year or so until I said to them that they had better take me to court to get the money they think I had not paid....... after that they gave up.
My guess it that once the accounts people handed it over to the legal people and the case was being prepared by the solicitors they found it necessary to check their records for the evidence to present to the court....... and found I had paid !
Quite an expensive way to sort out a minor accounting error....may be that's why they went bust ! ... a lot of cock ups like this .