1. Search for an existing constituent with that ID:
    1. Go to Records > Open a constituent. 
    2. Search using the Constituent ID to confirm they don't already exist. 
  2. Assign a different a constituent ID if it is a different constituent.
  3. Select Update existing records and mark the Import new records not found checkbox to update the existing and import new constituents in the same file.
  4. Make sure any constituent IDs you are assigning are 20 characters or fewer. If they are longer then 20 characters they will truncate which can result in non-unique numbers.