Select A1:A10, the use Data / Validation... on the "Settings" tab,
Select Custom under "Allow"
and use the formula

=COUNTIF($A$1:$A$10,A1)=1
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"B. J." wrote:

Is there a way to prevent duplication of numbers in Excel cells such as
Social Security Numbers. Can an alert be set to get notification when a
duplication is entered in another cell in the same colomn occurs?