Bugzilla had a neat feature intended to reduce duplicate bugs: when a user was creating a bug, Bugzilla would use AJAX search to take the bug summary and search through other open bugs that seemed similar. Bugzilla would present seemingly duplicate bugs to the user in a list, theoretically reducing the number of duplicate bugs filed.

Phabricator doesn't seem to make any effort to avoid duplicate tasks and it would probably be a good idea if it did.