Use this one-step tool to compare two columns on a single Google sheet. It detects duplicates and unique values that you can easily highlight with the selected color, mark in a status column, copy/move to another place or delete.

Please keep in mind that the add-on processes values in hidden & filtered rows as well.
If you want to match columns from multiple sheets or analyse multiple tables, use Compare Sheets for Duplicates instead.
You'll find this tool in all our collections: Remove Duplicates, Compare Sheets and Power Tools.
Run the owned collection from the Extensions menu:
There's just one window with all the necessary comparison options:

The dialog box allows you to pick your data in different ways:
Select the necessary cells and click OK to confirm the reference.
Hit Compare to start comparing two columns in Google Sheets.
When the add-on is done, you'll see a confirmation message saying how many duplicate or unique values have been colored and how they've been handled:

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