There's an example here for preventing duplicate entries with data
validation:

http://www.contextures.com/xlDataVal07.html#Unique

Or you can hide used items in the dropdown list:

http://www.contextures.com/xlDataVal03.html

rowing coach wrote:
is it possible when using a drop down menu to have an error message pop up
if you've selected something that has already been used. Example - if you are
making seating plans where you want to make sure you don't select the same
person twice.


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Debra Dalgleish
Excel FAQ, Tips & Book List
http://www.contextures.com/tiptech.html