"chrishocking" wrote:
I have an excel spread sheet into which serial numbers are input. However a
serial number can be duplicated, how can i stop this from happening.

You could try using Data Validation

Assuming serial nos are input into A1 down
Select col A, click Data Validation, make the settings as
Allow: Custom
Formula: =COUNTIF(A:A,A1)<2
Ok out

Test it out by trying to input some duplicate serial numbers into col A.
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Max
Singapore
http://savefile.com/projects/236895
xdemechanik
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