Repeated errors even after an initial letter of apology

A pensioner has been having injections in his eye for several years. This year Momentum have made errors in the account 3 times. They say the service provider has duplicated a code but the code is correct and needs to be billed twice. A letter of apology from Momentum was sent in March but the same error was made the following time and again the third time. It is time consuming to continually have to phone and sort out errors. Please can this be given your urgent attention.