ref: 279f78a86cbff3f526fe1a1debd4d4f5766f96b0 dir: /test/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" }