Using data validation you can; but only manual entries/dropdown entries will
be validated

--Select B2:B10
--Select menu DataValidation
--Select Custom from the 'Allow' dropdown and enter the below formula

=COUNTIF($B$2:$B$10,B$2)=1
--From the Error alert tab enter the message you want to be displayed
--Click OK and try.

If you are selecting the entire column the validatio would be
=COUNTIF(B:B,$B$1)=1


If this post helps click Yes
---------------
Jacob Skaria


"Michelle" wrote:

Hi!

How to set data validation function (in column B) to restrict user from
entering same invoice number from same supplier?
Column A: Supplier
Column B: Supplier Invoice Number

TIA