[vcmiraldo]

Copy just copies once. The need for duplicate is clear if you're trying to diff something like `t = [a]` and `u = [a, a]`. You could copy `a`, but you'd have to decide whether to copy it on the first or second position; the second one would be classified an "insertion" by any ins/del/cpy-algorithm. If you instead opt to NOT make that choice, you can say: pick the source `a` and duplicate it instead