I think both the old and new diff worth a look. Maybe showing them using different colors is a good idea.

Phabricator uses light colors to show changes not introduced in this patch but a rebase against master [1]. The experience is pretty good if the workflow is to always fast-forward and the patch author does rebase. Changes introduced by other people are in light green and red. Duplicated-line issue can be obviously seen.

[1]: https://secure.phabricator.com/book/phabricator/article/diff...