I want an error to show.

Hello,
Im a newby to excel and learning formulas at present.

-I have a column in spread sheet.
-The purpose of this column is to enter a product code on each row of this column.
-The column in theory should only have each product number entered once.
-So i am trying to find a formula to avoid any code being entered more than once.
-As we are visually checking this column to make sure that the product code is only entered once, we have had a few instances of human error, so i am looking for a formula that will highlight the code in red if it has already been entered ( so highlighted in red will be both the same codes, so visually we will be able to identify code entry error)

- I hope this is clear as im new to excel. Any help would be much appreciated.

Cheers

Joel