data validation not working while we paste the values (for removing duplicates in a column

Hi,

I put a data validation formula for removing duplicates in a column (ex: data validation formula is :- =countif(D:D,D1)<=1). While we entering values in D:D column, the formula is working. when we copy and paste the values in D:D column, data validation is not working. Pls help on this?

Thank you in advance.

Regards
Ajay Pal