Use cell validation, which you can read about or view at
Validate / Validation
http://www.mvps.org/dmcritchie/excel/validation.htm
=COUNTIF(tablename,A1)=1

Data Validation lists, Debra Dalgleish, Contextures.com
http://www.contextures.com/xlDataVal01.html

a video presentation look for " - Data Validation in Excel" at
http://www.datapigtechnologies.com/ExcelMain.htm

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HTH,
David McRitchie, Microsoft MVP - Excel [site changed Nov. 2001]
My Excel Pages: http://www.mvps.org/dmcritchie/excel/excel.htm
Search Page: http://www.mvps.org/dmcritchie/excel/search.htm

"JOHNNYG" wrote ...(translated to English)
I am working with a list of reference numbers and want to stop duplicate entries