Alert when entering duplicate number?

[hyewiz]

Hello;

I have a workbook that i keep track of our purchase orders, what I like to have is away that when entering the new Purchase order number to check the entire workbook and Alerts me when there is the same purchase order number entered before.

thank you

 

[hyewiz]

Thank you very much for all your efforts to help me

I guess I am not able to explain what exactly I need

let me try like this. lets say I started a new book, in sheet 1 I want
a data to alert me lets say with a color change every time I enter
the same number ex.(PO001234) in any cell in any worksheet in that particular workbook.

Sorry if I am being a bit newbie to you guys, please accept my apologies.

Thanks​