ref: 2a3ee1c2e3e1ea6477ca8715bf1531dbf34367a1 dir: /testdir/t.index/
{ n = length d = 0 for (i = 1; i <= n; i++) if ((k = index($0, substr($0, i))) != i) { d = 1 break; } if (d) print $0, "has duplicate letters" }