Dear expert,

I want to set a cell (for example : A1) with validation so that the value in
it will not appear twice in a row, say A1:M1. My formula is:
=COUNTIF($A$1:$M$1,A1)<=1.
The above formula is working but I want the same validation also working for
Column A, say A1:A10. The PC blocks me and it states a cell cannot be set
more than one validation. Can this be done? If yes, what the formula will be?
Please advise.

Thanks in advance.