Related comments upstream: https://secure.phabricator.com/T4984#96400
It seems that duplicate panels are being created in Dashboards by users that either don't know how to add existing panels or don't fully understand how Phabricator's Dashboards work. This may be a feature that should be reported upstream but it would benefit the organization of this site if potential duplicate panels would perhaps show a dialog with existing panels that the user should add instead of creating another with the same purpose. This is quite a trivial issue and shouldn't be worked on at this moment since we are in the middle of a major migration. Just something to look into.