You are not able to save the export definition for an appeal mailing when multiple paths have the same custom name for the column header. For example: in the image below, two different paths were edited to have the name "Constituent 1", which caused the error to show when saving.
  1. In the export definition find the paths that were edited to have the same column names, and click to highlight one
  2. Click the pencil icon at the top of the Selected Fields section
  3. Edit or remove the custom name so that it doesn't match the same custom name as the other path
  4. Save
User-added image