Document & is already allocated to exchange &

What causes this issue?

Document &V1& is already assigned to Exchange &V2&.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Check if the assignment of Document &V1& to Exchange &V2& is correct
and remove the assignment of the assigned document, if necessary.

Error message extract from SAP system. Copyright SAP SE.