Let's say your data in Column A
Select column A:A
Go to menu, Data Data Validation under Allow: select Custom under
Formula: =COUNTIF(A:A,A1)=1



"Jan Buckley" wrote:

Is there a way to identify an entry as a duplicate in the same column. For
instance, if 4310011192 is already entered in the column and I enter it
again, an error will result, informing the user "Duplicate entry". I know you
can do this in ACCESS by virtue of the primary key, but don't know how to do
it in Excel.