As above, it depends, it depends.

At the very simple level, you could always open the CSV in Excel, use the Data drop down, Filter, Advanced Filter.
On the Advanced Filter window, you can choose to filter your data in place, or copy to another location - and there's a tick-box (check-box) marked 'Unique records only'. Click it.
Select, or type in, the data range that you want to filter, then give a location for the filtered data ($a$1:$a$999 filtered to $c$1:$c$999).
Done
De-duped in one pass, and especially useful for email lists.