Dedup authors not graduated summary data

[jazairi]

Why these changes are being introduced:

Theses with multiple files are being counted for each attached
file in the authors not graduated summary data.

Relevant ticket(s):

https://mitlibraries.atlassian.net/browse/ETD-429

How this addresses that need:

This dedups theses to fix the count in the Report index.

Side effects of this change:

N/A.

Developer

• All new ENV is documented in README
• All new ENV has been added to Heroku Pipeline, Staging and Prod
• ANDI or Wave has been run in accordance to
  our guide and
  all issues introduced by these changes have been resolved or opened as new
  issues (link to those issues in the Pull Request details above)
• Stakeholder approval has been confirmed (or is not needed)

Code Reviewer

• The commit message is clear and follows our guidelines
  (not just this pull request message)
• There are appropriate tests covering any new functionality
• The documentation has been updated or is unnecessary
• The changes have been verified
• New dependencies are appropriate or there were no changes

Requires database migrations?

NO

Includes new or updated dependencies?

NO

[coveralls]

Coverage remained the same at 97.17% when pulling 2fab76c on etd-429-author-not-graduated into f62093c on main.