Two defective chains may appear mutually supportive while deriving from the same error, the same unreliable authority, or a common intermediate. Such non-independent repetition does not equal two separate witnesses.
Moreover, proved fabrication, deliberate falsehood, and severe unreliability are not equivalent to mild deficiency in memory. A defect too serious to be offset by sound corroboration does not disappear merely because additional derivative versions exist.
Example: if three pupils reproduce the identical mistake of one weak transmitter, the lower branches multiply but the original source of the error does not. This is why the common pivot of an isnād must be identified before treating the routes as independently corroborative.