Item &1 already has a schedule line for date &2 &3 &4

What causes this issue?

A schedule line with the same date and the same time was already
created for item &V1&.

System Response

You receive a warning message because this does not usually happen.

How to fix this error?

Check whether you really want to create more than one schedule line for
date &V2& &V3& and time &V4&.

Error message extract from SAP system. Copyright SAP SE.