You are unable to add any additional fields in canned reports in Altru, so there isn't a way to see the date the constituent records were created within the Duplicate Constituents report. However, you can pull up the individual constituent records to see the date added with these steps:
  1. Click on the name you would like to check the date for on the Duplicate Constituents report
  2. On the left side of the page under Reports, click Constituent Profile
  3. With the Biographical Summary box checked, click View Report
  4. See the date that the constituent record was created in the Date Added section