No formulas required. This works on as many columns as you need, but will only compare columns in the same worksheet:
NOTE: remove any duplicates from the individual columns first!
- Select the columns to compare
- click Conditional Formatting
- click Highlight Cells Rules
- click Duplicate Values (the defaults should be OK)
Duplicates are now highlighted in red
- Bonus tip, you can filter each row by colour to either leave the unique values in the column, or leave just the duplicates.