A payment document could not be added to the document buffer

What causes this issue?

You wanted to add a new entry with ID &V4& for bank &V1&, account &V2&,
during printout with sequence number &V3&. This was not possible,
because an entry already exists.
This entry cannot be made with mass assignment of external check numbers
to document numbers.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Check the reason for the error, and after elimination execute program
SAPFKPY3 to start a new payment medium run.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.