For a "marked as duplicate" timeline event to appear, the user who creates the duplicate reference comment must have write access to the repository where they create the comment.
To mark an issue or pull request as a duplicate, type "Duplicate of" followed by the issue or pull request number it duplicates in the body of a new comment. You can also use the GitHub-provided "Duplicate issue" or "Duplicate pull request" saved replies to mark an issue or pull request as a duplicate. For more information, see "About saved replies."
You can unmark duplicate issues and pull requests by clicking Undo in the timeline. This will add a new timeline event, indicating that the issue or pull request was unmarked.