This error is due to the export definition of the acknowledgment process containing any duplicate merge fields. This is not specific to Email Address, Finder Number or Constituent ID- this error occurs due to ANY merge field duplicates.

The error message mentions CONSTITUENTID because it always has to be included in the Export Definition in the default location and with the correct alias (first “Constituent Marketing Information” node). Same for EMAILADDRESS and FINDERNUMBER.

To find the merge fields, please use following steps:
  1. Navigate to Marketing and Communications > Acknowledgements
  2. Next to any letter click drop down and select Edit
  3. Next to Merge fields, click Preview
  4. Copy/Paste those fields listed into Excel
  5. Alphabetize the list to find the duplicate merge fields

To remove the duplicate merge field from export definition:
  1. Navigate to Marketing and Communications > Acknowledgements
  2. Next to any letter click drop down and select Edit
  3. Next to Merge fields, click Edit
  4. Within Selected Fields highlight the duplicate merge field
  5. Click the red X to delete the line item
  6. Save and close