slect the column where the user enter the data
then go to Data--validation-- and select Custom in the drop down choices
and type this
=COUNTIF($E$1:$E$1000,E1)=1
if Column E is being used to enter the info by users

it will give an error message anytime an entry is repeated

"Ru" wrote in message
...
I have a list of contract numbers relating to application numbers or
payments. they are in the format nnnnnnan or nnnnnnpn. The columns are
fixed
to this format only. If they are entered with the a or p in the wrong
place
or if they have been left out completely an error message will appear to
alert the user. I want to know how to alert the user if they enter an
application or a payment number that has already been entered.
ie if they enter 022079a4 but that same application has been entered else
where in the column.
If anyone has a step by step suggestion that would be very helpful. And if
its a formula should i put
it at the top of the column or where. All suggestions gratefully accepted.
Hope you can help