$ cat index_DATA
1   32
4   19
28  39
34  123
321 321
324 329
325 349
340 509
$ perl -nle's/(\d*)(\d*)\s+\1(\d*)/$1$2-$3/;s/-$//;print' index_DATA
1-32
4-19
28-39
34-123
321
324-9
325-49
340-509
$