I tried the script and its printing all the rows

Then you have to show me your input data. I've just tried that script with the following input data:

aa blah
bb blah
bb blahblah
bb foo
cc dlqskjf
cc cfkqs
dd dkls
ee dsjkqjs
ff blah
gg klsqdj
gg sqkl
[download]

and it print only the lines where the first column is a duplicate, as shown in this output:

bb blah
bb blahblah
bb foo
cc dlqskjf
cc cfkqs
gg klsqdj
gg sqkl
[download]

This seems to work perfectly.

Otherwise, the way it works is that it reads the file one line at a time, and store this line ($previous_line), as well as the comparison key until the next line is read. If they have the same key, then I print the previous line (if defined) and the current one; in such case, I undef the previous line to prevent it from being printed twice if there are triplicates.

If it does not work properly for you, please show your input and/or test data.