Suppose one chain runs Ahmad ← Malik ← Nafi‘ ← Ibn ‘Umar, while another runs Khalid ← Malik ← Nafi‘ ← Ibn ‘Umar. For Ahmad, Khalid provides a complete mutaba‘ah, because both directly transmit from Malik.
The report can no longer be described as uniquely transmitted by Ahmad *from Malik*. Nevertheless, the additional route must still be evaluated for its own reliability and possible error.
The operational question is: Does the supporting narrator share the very same immediate teacher? If yes, the suspected singularity at that point is answered by a complete mutaba‘ah.