An allocation already exists

What causes this issue?

The material component that you want to allocate to the operation is
already allocated to another operation of the standard sequence.
You can only allocate the material component again if the new
allocation is to an operation of an alternative sequence. This sequence
must be an alternative to the operation of the standard sequence, where
an allocation has already been made.

How to fix this error?

Allocate the material component to an operation of an alternative
sequence or
Allocate another material component.

Error message extract from SAP system. Copyright SAP SE.