If i have a range of cells from A1:A100 and the same drop down list of names
in each of those 100 cells how do i prevent staff from selecting the same
name twice within that range?

For example i want to avoid someone selecting a name in cell A3 and then
selecting the same name further down the spreadsheet at say A70.

Is it also possible to display a message saying that a duplication has been
attempted?

Thanks in advance
Craig