Interestingly, there seems to be some disagreement as to know the Duncan Loop is tied. This version clearly shows a slip-knot:
Yet I just watched a video that shows the initial loop being made in the standing part, which of course would prevent the knot from slipping:
However I don't know if this knot has the same strength as the slipping version. Actually I don't know if the slipping version has the strength that is claimed in the OP, but I have heard 50% for the bowline many times.