Runtime error MESSAGE_TYPE_X program SAPLMBWL

Short text of the error message:

User PRDEWMUSER already processing Purchasing document 4503504332

Long text of the error message:

Technical information about the message:

Message class....... ME

Number.............. 006

Variable 1.......... PRDEWMUSER

Variable 2.......... Purchasing document

Variable 3.......... 4503504332

Variable 4.......... " "