Purchase order mail sent twice in SOST

Hello,

do you know why sometimes the mail created for sending Purchase Orders to the vendors is sent twice in SOST? They are new purchase orders, they don't have any update, in the NAST table and transaction ME9F only one record is displayed as in the message log of the PO.

In addition, the PDF document sent to the vendor are exactly the same, other one of the PDF is without PO number or without the company logo.

Thank you in advance.