To prevent duplicate entries use 'Validation' from menu Data

1. Highlight the range, lets say A1:A100
2. From menu choose DataValidation
3. Under 'Allow' drop down choose 'Custom'
4. Enter the following formula in the Formula box
=COUNTIF(A$1:A$100,A1)=1
5. You can set a Error Alert if you need.



If this post helps click Yes
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Jacob Skaria


"John" wrote:

I have a spreadsheet running that includes the scanning of a serial reference
number containing text and numbers on software. I need to secure that column
to prevent duplicate numbers being scanned and entered into the sreadsheet.
This then preventing dupilcate serial numbers being attached to software
before shipping. I've looked at condition formats and validation but can't
see the way forward yet.