Problem with select for update skip locked

   Hi all,

We have an outbound interface like this:

-Step1: cursor Select invoice_id, amout from ap_invoices_all using

FOR UPDATE OF invoice_id skip locked

FOR UPDATE OF invoice_id skip locked

-Step2: validate some informations in cursor's data.

-Step3: Insert those data into customized table.

-Step4: commit.

 

The problem is:

When two request submit the outbound interface at the same time(HH:MI:SS) then one invoice is duplicated in customized table.

This is a big probem so I want to ask what wrong with this? How we can avoid it?

(Another way we can use constraint