SRM SC backend error : ME 218 External document number XXXXX already assigned.

SRM Experts,

We are facing a problem in our system for SCs.

We have the classic scenario with a SRM 7.01 and ECC 4.5B version.

The shopping carts in SRM randomly fail with backend errors some of the shopping carts fail with back end error "W5 107" and when it is reprocessed in application monitors, the error "ME 218 External document number XXXXX already assigned" is shown.

The PO document is created in the backend but the SC document is not updated with the backend document link.

It always shows the error message "ME 218".

we are unable to process these carts.

Need your suggestions for proceeding further.

Thanks in advance.