Date & not valid for scheduling periods, overlapping

What causes this issue?

You entered schedule lines which have the same scheduling date.

System Response

When you copy lines in the qty.schedule, the system copies info. on the
date as well. That means that you have to change the date manually.
The error only occurs in this specific situation.

How to fix this error?

Avoid duplicate scheduling periods by changing the date.

Error message extract from SAP system. Copyright SAP SE.