One easy monitor/alert is to use a formula in an an adjacent col
Assume you are entering data in A2 down
Put in B2: =IF(COUNTIF(A$2:A2,A2)1,"<<Dup !","")
Copy down to cover the max expected extent of data in col A
Col B will give you the necessary visual alerts

Works ok? Click the YES button below
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Max
Singapore
http://savefile.com/projects/236895
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xdemechanik
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"KKD" wrote:
I have rows of data that contain a column with an invoice number, if I am
entering a duplicate invoice number, I want to be alerted. I want to be
allowed to enter duplicates, but I want Excel to somehow alert me that it is
a duplicate so I can check it. What is a good way to do this?
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KKD