I'm looking to color a cell only when duplicates are entered.

If I enter 645 in A3, then enter the same # in A4 I want A4 to display a color when I drop to the next cell. I used 645 as an example, so I guess it's better to say that A4 shows color when he same # is in A3, A5 when the asme # is in A4 ect.