Shift group not unique

What causes this issue?

The shift group assignment is ambiguous in the period specified. Either
the shift group is not assigned to the organizational unit for the
entire period, or more than one shift group has been assigned in the
period.

System Response

It is only possible to edit a period in which the shift group
assignment is unique (i.e. not ambiguous).

How to fix this error?

Choose another period in order to continue editing.

Error message extract from SAP system. Copyright SAP SE.