identify duplicates

how do I identify duplicates? Not just remove them.
I need to know which numbers are duplicated.

I have a list of numbers like this:

015-2066-000-0P
015-2067-000-0P
015-2499-000-0P
015-2500-000-0P
03-3466

I need to know which are dups not just remove them.

thanks.

One quick and dirty way to get a glance of duplicate rows would be to
- make a two copies of your document
- 1.: Edit > sort *without* duplicate removal
- 2.: Edit > sort *with* duplicate removal
- text diff with active tab from (inactive) tab right mouse click context menu.

First step is obsolete, it sort lines only.
Your second step will sort lines first and than remove duplicates in one step.

I meant "first" simple sort step for first copy and "second" sort with duplicate removals for second copy, to get differences of the two.