Dont want same person to be booked for same shift twice

Hi all,

I am designing a excel sheet which is meant to help us manage day and night shifts for several people at different sites. I have a drop down menu in cells in the left most column to select from a list of names and then 7 column on the right for each days with day/night dropdown option with color formatting for days and nights. My problem is, I don't want it to be possible for someone to be booked for same shift in two different rows and I don't know how to achieve that. for example if mr A is booked for Monday day shift, it shouldn't be possible to book him again in a different row for the same shift.

can anyone help me with this? I have attached a screenshot of the file here.

Thanks
list.jpg