Prevent Duplicate Records

[Freshman]

Dear experts,

I've a table. Colum A is for account numbers. I set a formula in
"Validation" to prevent duplicate records. This works well whenever I input
numbers. However, when I copy a number from another workbook and paste it to
column A, the formula does not work even the number is a duplicated number.
In this regard, please advise how to remedy this problem so that duplicate
numbers pasted are rejected by the formula in column A?

Thanks in advance.