If one route reads “A from C” while another independent route reads “A from B from C,” the second route can help investigate the first. But the longer form is not automatically correct: one must determine whether the additional intermediary is preserved or an error, and whether the routes share the same origin.
The shorter route may involve abridgment or tadlis; alternatively, the longer route may contain a mistakenly inserted transmitter. Neither assumption should be made without evidence.
Method: identify the common source, compare transmitter reliability, inspect explicit hearing, and consult critical evaluations. Only then determine which chain is better preserved and whether an omission or addition is established.