Check out this web page of Chip Pearson:

http://www.cpearson.com/excel/NoDupEntry.htm
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HTH,

RD
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Please keep all correspondence within the Group, so all may benefit!
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"Bonnie" wrote in message
...
Is there a way to prevent the entering of a duplicate
number in a column. For example, if you are entering
Invoice numbers in Column A, is there a way to prevent
the entering of duplicate Invoice numbers? If so, explain
it.

Thanks.