double data

how can i determine if i have a double data in whole column?
is there a validation to prevent double data entry?

tnx

To use data validation to prevent duplicate entries in Column A, say. Highlight the column and enter the following formula in the appropriate Data Validation box:

=Countif(A:A,A1)=1


David.

formula is working in typing data only
but when i copy paste validation is not working is there another way?