I am reading Matthew closely as literature, and was curious whether Jesus' ethical teachings (do unto others, etc.) were a synthesis of earlier systems of thoughts, or partly/wholly original -- at least as far as we know.
I've been rereading some Plato recently, so I have one example to give. In Crito Socrates argues for a pacifistic position. He reasons that harming another, no matter what the situation, harms one's own moral self (he considers one's moral self a distinct aspect of one's being). Thus, even in retaliation it is wrong to hurt another person. Not quite "turn the other cheek", but it is radical pacifism. That said, this only apples to interpersonal conflict, Socrates himself was a military veteran. Again, only one small example of a similar teaching, but I hope that helps.
I've cross-posted your interesting question to the new /r/AskBibleScholars.