Validate Header Text (MKPF-BKTXT) in Migo

Hi experts, I'm new in SAP OO.

I need to validate the header text in Migo. We put in this field the number of invoice of goods receipt. Users sometimes make mistakes and re-enter the same invoice number, making problems in the stock. I need don't duplicate the register of this invoice.

I tried to implement BADI MB_DOCUMENT_BADI Method MB_DOCUMENT_BEFORE_UPDATE but I don't know how to do it.

Or exist other way to do it?

Please give me, if possible, some example.

Thanks in advance.

Hugo