I was having a discussion with my father on this passage, and he shared with me an interpretation that there are "two cups", one "before supper" and one "after supper" (verse 20).
The one before supper, he contends, is a Passover cup (Old Covenant), while the second cup is the New Covenant, the blood. I haven't found any patristic commentary that makes this distinction. So it seems to me there are not two cups, but just the same cup mentioned twice. Am I wrong to think that this is the same cup repeated a second time?