The Harmony of the Gospels
The Diatessaron of Tatian is translated with notes by Hill, The Earliest Life of Christ. See also Ante-nic. Fathers, IX. 35-138.
For the extreme position concerning Doublets see Holtzmann, Hand-commentar zum NT I. passim. E. Haupt, Studien u. Kritiken, 1884, 25, remarks that Jesus must often have repeated his teaching in essentially the same form.