github.com/benhoyt/goawk@v1.8.1/testdata/t.index (about)

     1  {	n = length
     2  	d = 0
     3  	for (i = 1; i <= n; i++)
     4  		if ((k = index($0, substr($0, i))) != i) {
     5  			d = 1
     6  			break;
     7  		}
     8  	if (d)
     9  		print $0, "has duplicate letters"
    10  }