Select the column, assuming D1 is the active cell, for example, do this:
Use Data/Validation, select Custom from the first dropdown, enter this in
the Formula:
=Countif(D:D,D1)=1
then set the appropriate error alert in the error alert tab.

Bob Umlas
Excel MVP

"Clearwater Tim" <Clearwater wrote in message
...
I am trying to set a check register example and want to prevent the user
from
entering the same check number twice. Is this possible in Excel?