You cannot carry out deliveries for the same sched.agreemt more than once

What causes this issue?

A scheduling agreement that is already contained in an existing
deliveryto be added to this delivery.

System Response

This is not allowed. Only other scheduling agreements or orders can be
added.

How to fix this error?

To deliver a greater quantity against this scheduling agreement,
increase the quantity of the corresponding delivery item.
To deliver other items of the scheduling agreement, create a new
delivery.

Error message extract from SAP system. Copyright SAP SE.