How to use validation rule within database with drop down menus.

Building a scheduling database using access 2013 for my company. I currently have it set up so that only employees that have a rating of 3 or more for the particular job show up when you use the drop down menu. However since we have several associates that meet that criteria for multiple positions there is the risk that the supervisors may double schedule someone. I am wondering if there is a way I can have a dialogue or warning pop up if the same name is added to the schedule in two places. If you need more information please let me know! I feel like this is way easier than I am making it.