Unique Invoice Number

[Guest]

I enter invoice numbers (alpha/numeric) in one column
titled Invoice Number. My question is: Is there a way
that Excel can tell me if I have already entered the
invoice number as my file grows? I do not want the same
invoice number entered more than once on my spreadsheet.

Please help.

 

[NHarkawat]

Select column A
then Using data validation under custome type the formula:
=countif($a1:$a50000,a1)=1
It will give an error message if a duplicate invoice numer is re-entered

 

[NHarkawat]

Make the formula absolute as follows:
=countif($a$1:$a$50000,a1)=1

 

[sphenisc]

Put in a conditonal format for the whole column

e.g if it's column B

Under Format menu select Conditional formatting
Change 'Cell Value is' to 'Formula is'


=COUNTIF($B$1:$B1,$B1)>1

and select a suitable warning format e.g. red background

 

[Frank Kabel]

Hi
also see:
http://www.cpearson.com/excel/NoDupEntry.htm
to prevent such entries