I have a workbook where I need the user to be able to enter only one entry
for a two column pair.

Column A is an identifier number and Column B is a date, so:

#1, 10/16/2009
#1, 11/16/2009
#2, 10/16/2009

is valid but,
#1, 10/16/2009
#1, 10/16/2009
#2, 11/16/2009

would not be valid.

I've thought about concatenating these two fields in another sheet and then
doing a vlookup on this table and if it is N/A it would mean it's not
previously used, but that seems clunky...As well as doing a loop to determine
if it's been used since this sheet can have thousands of entries.

Is there a simple way of doing this that I might be missing?