Option 1: Unmark the 'Use field names for one to one column headers' on the export definition's properties, generate a new header file, create a new merge letter, and then edit the existing receipt process or add a new one.
Option 2: Change the names of all of the fields in the export definition (even if they appear to be unique), generate a new header file, create a new merge letter, and then edit the existing receipt process or add a new one.

For more information, see Create an export definition