>> If we don't want new file we can redirect the output to same file which will overwrite original file

You need to be a little careful with that. If you do:

    uniq -u movies.csv > movies.csv

The shell will first open movies.csv for writing (the redirect part) then launch the uniq command connecting stdout to the now emptied movies.csv.

Of course when uniq opens movies.csv for consumption, it'll already be empty. There will be no work to do.

There's a couple of options to deal with this, but the temporary intermediate file is my preference provided there's sufficient space - it's easily understood, if someone else comes across the construct in your script, they'll grok it.