Avoiding Double Entries on Worksheet

Hello,

I have a worksheet that I use to populate manpower to specific jobs (I have attached it). I use data validation to make the inputer pull the person from a list of names.
The problem I have is that the same employee is being allocated to multiple jobs for the same day. By the time it is noticed it creates a scheduling problem.

So this is what I thought would work: If a person puts an "X" in a box that is next to a name, and if that person already has an "X" on that date for another job, then a warning box would pop up saying "This person is already scheduled for another project on that date. Area you sure you want to confirm this double booking?" Then there would be a yes or no box to click. Another thing that I thought would be cool is to have a "show me" button where it would go to the other cell that has the person checked, but this is not necessary.

Can anyone here help me out with that? It would definitely make my life easier.

Thanks!
Seth