Alert Me Upon Entering Duplicate Number

Is there a way to have Excel alert me (via cell color change preferably, but another method would be fine also) upon entering a number that has already been entered on that worksheet?

The problem I have:

I have 10 columns in a worksheet that I keep track of serial numbers in for work... Sometimes someone enters the wrong serial number or doesn't update a serial number so we will have the same serial number in two different places and another agency will call us to correct our mistake, making us look inept.

I figure the best way to alert people of this input error would be to change the fill color of both, or the active, cell(s) so they could immediately begin looking into the problem.