Transactions are considered duplicates if for a customer it already exists in either the current, historical or matching files. It may be that this document number happens to exist for this customer, just out of coincidence, in any of these files. Print the Customer Transaction report for just that customer, selecting to print both Current and Historical transactions, and also select the 'applied detail' option to show the information in the Matching file.

As the error message occurs when posting, the procedure is finding a duplicate somewhere for that customer.

One known issue that may be relevant - a problem occurs in posting if a customer does not have any applied transactions, and the next customer in the list does and has that same check number already posted. Then posting is reporting a duplicate when it shouldn't. The workaround for this (until the next service pack for Receivables is released) it to simply change the document number, such as add an A at the end.