Google Sheets is one alternative to Microsoft Excel.
This is something that can be done quite easily if you are using Microsoft Excel, but for Google Sheets, it is a bit more complicated.
- Highlight the column you want to check
- In the menu, select Format | Conditional Formatting
The "Conditional format rules" menu will appear on the right hand side
- Select the range if you have not already
- Under "Format rules Format cells if...", select "Custom formula is"
- Type in:
=countif(A:A,A1)>1
- Under "Formatting style", choose the format you would like.
Click on the word "Default" to select one of the default formats.
- Click the "Done" button