Hi,

Select column A and then

Data|validation

Select custom and enter this formula

=COUNTIF($A:$A,A1)<2

Mike

"Malcolm McMaster" wrote:

I am having a problem with a client using Excel to record receipts issued
to her clients.
Looking at the data she has entered it appears that she has entered some
receipts twice. Is there any way one can enforce unique integrity on a data
column so that if the operator tries to enter the same number it will be
rejected or ring warning bells ? If you can offer any suggestions it would be
much appreciated.