At the Last Supper did Jesus pass the cup once, or twice?

Matthew and Mark: Once (Matt. xxvi, 26–30; Mark xiv, 16–26).

Luke: Twice (xxii, 13–20).

Regarding this discrepancy, Scribners’ “Bible Dictionary” says: “The temptation to expand was much stronger than to contract; and the double mention of the cup raises real difficulties of the kind which suggest interpolation.”