Drop Down with no duplicates in the cells

[xTrashbandicoot]

I'm creating a work schedule and currently the excel allows me to select from a drop down using data values. What I'm looking for is a way (formula) that recognizes a duplicate in the cells I have listed below. Yellow is Names drop down (don't care about duplicates there. White has drop down for job position (this where I cannot have duplicates). Please help!

 

[Jimmypop]

Hi there

Where is the White that ha drop down...is it all the cells in colomns D to N? Also what needs to be done with the duplicates.. do you want to just get a warning or do you want to prevent duplicates... Please post sample of eht it should look like...