Highlight the range say A:A if it is in column A
goto data, validation,custom
enter this formula in the box
=COUNTIF(A:A,A1)<2
then go to the error alert tab and enter the message you want, when a
duplicate number is entered


here's a little clip on data Validation

http://www.datapigtechnologies.com/f...alidation.html


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davesexcel


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