A chain has six narrators. The first four use “samiʿtu” at each step; the fifth link lacks this attested wording, and the sixth again uses it.
The repeated formula cannot be confidently claimed for the entire chain when one link is unproven. The more accurate description is verbal seriality in a portion of the chain.
In another case, all six reliably use “samiʿtu,” but the third narrator has seriously weak retention. The serial feature may be established independently, yet the hadith cannot be called authentic merely on that account.
Thus proof of a repeated feature and authentication of the report remain different conclusions.