What I mean is it seems to work for all of column A by using data validation
and custom =COUNTIF(A:A,A1)<2. But if I want to do a section of the columna
say A50:A60 I can't get it to work.

"Satraj" wrote:

Say if I had the folloing data in column A1:A3 and I try to enter 300 in A4
again, using data validation I want to be able to be stopped from entering
that value again.
Column A
200
300
400