Component &2 was skipped during BOM explosion of &1

What causes this issue?

The BOM &V1& has multiple input components for product &V2& defined. In
SPP, BOM's cannot have multiple duplicate components. In such a case,
only the first component is taken into account from these duplicates.

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

System Response

Component &V2& item &V3& has been skipped.

How to fix this error?

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.