This error is generated when the exact same name is in use on another email. Because the name entered could be on another part, the system will not allow a duplicate entry. To resolve you can perform either of the following: 
  1. Alter the desired name slightly to be different then what is being entered
    • Example: Instead of using "Test Acknowledgement" as a name, try adjusting the name value to "Test Donation Acknowledgement"
  2. Locate the existing part where the same name exists and edit/update the name to a different value.