Try this in that 'helper' column to the "right" that you mentioned:

=IF(AND(A1,COUNTIF($A$1:A1,A1)1),"Duplicate","")

And copy down as needed.
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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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"rufusf" wrote in message
...

Hi
I am making a spreadsheet where I am inputting a large list of
different numbers - I want to put a check in to make sure no number is
input more than once.
What would be the easiest way to do this? (ideally a column to the
right of the number could be used to flag up where there is a duplicate
number)

Thanks




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rufusf