When adding a new name format to a constituent record, ensure that the type that you select in the drop-down isn't already being used on an existing name format for that constituent. You can either select a type that isn't currently being used, or edit the existing name format type to choose the format you would like. 
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To edit an existing name format:
  1. Search for and open the constituent's record
  2. Select the Personal Info tab and scroll down to the Name Formats section
  3. Click the drop-down arrows next to the name format with the type that you want to change
  4. Click Edit
  5. Select a new name format and click Save