This means that the customer of the payment and the customer of the invoice is not the same.
For example
The customer of the payment:

In MD: number 1557, name: West Coast Automatics
In MYOB ID: 30820893, name WESTCOAST AUTO'S"

The invoice in MYOB:
Customer: West Coast Automatics, but this one syncs to a Supplier named West Coast Automatics in MD
There must be some sort of merging in MYOB that caused the invoice to have the contact that is actually linked to a Supplier in MD.

In Short

You have a supplier and customer with same name in MD West Coast Automatics and two contact in MYOB
WESTCOAST AUTO'S  and  West Coast Automatics

The question now is, are they the same one and if so, which one do they want to keep active


Once they've decided We'll fix up MD accordingly for them.