Limit a cell list option for more than one cell in the same column for two different rows

Hello,

Thanks in advance for looking.

I have a two page workbook. Sheet 1 is a staff schedule list with each employee listed in column A. The list of employees are divided by groups depending on their positions - bartenders, servers, hosts etc. Some employees are listed in more than one group as they are able to work various positions. Columns B through N lists the shifts available each day (Mon to Sat Lunch and Dinner, Sun Dinner). A pull down list is the main preferred option to select available times for the employee depending on the their position. Scheduled time options can vary between positions.

Duplicate Shifts.jpg

How do I prevent from assigning Ali - who is already scheduled to work Monday morning as a host and 9 AM - to another shift as a bus boy at 11:30 AM on that same Monday shift?

Thanks in advance!

Here is the workbook for reference. Thanks.