Idoc del. schedule number &1 is identical to the old del. schedule number

What causes this issue?

The received IDoc contains both the current delivery schedule number of
this scheduling agreement an a new, future delivery schedule number.
These numbers are the same.

System Response

This process is set in the status records of the IDoc.

How to fix this error?

Check the values entered for the delivery schedule numbers in the IDoc,
correct these and schedule this IDoc again.

Procedure for System Administrators

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

Error message extract from SAP system. Copyright SAP SE.