The control data is already being processed by &

What causes this issue?

DEFINE &USER& = &V1&
INCLUDE MESSAGE_ENQUEUE_USER OBJECT DOKU ID TX
You want to process plan or control data that is already being processed
by user &V1&. To prevent the data from being overwritten, it is locked.

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?

Wait until the locked data has been released again.
Alternatively, contact user &V1&.

Error message extract from SAP system. Copyright SAP SE.