Material &1 already exists in &2 &3

What causes this issue?

Material &V1& has already been created as an interchangeable material.

System Response

Material &V1& already exists in &V2& &V3&.

How to fix this error?

Check and if necessary, correct the entry.

Procedure for System Administrators

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

Error message extract from SAP system. Copyright SAP SE.