I had some questions about weak hadiths:
1. If a hadith has a weak chain of narration but there is another authentic hadith on the same topic or which has the same meaning, can the weak hadith be considered corroborated due to the sound hadith?
2. Is this from the methodology of the Salaf (pious predecessors)?
3. Can it be Hasan li-Ghairihi (Hasan [good] due to external factors) even if the Salaf did not say so about it?
4. For example, if a weak hadith has been said to be weak by the Salaf but there are good/authentic hadiths on the same topic and a later scholar says that it is Hasan li-Ghairihi, are they going against the grading of a scholar from the Salaf?

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger.

If there is an authentic hadeeth on a given topic, it cancels the need for the inauthentic hadeeth on the same topic. This inauthentic hadeeth is not considered authentic merely because its meaning was established in another (authentic) hadeeth. However, such inauthentic ahaadeeth may be cited as shawaahid (plural of shaahid, i.e. a hadeeth with a closely similar meaning reported from a different Companion through a different chain of narrators). Only the authentic hadeeth is accepted as evidence, and not the weak one. Hadeeth scholars were less stringent regarding the narrations of weak reporters as Shawaahid. Ibn Katheer said, “Narrating weak reports is tolerated and acceptable as far as the Shawaahid are concerned, unlike the case with the narrations about the fundamental rules of the Shariah, and this is the approach adopted in Saheeh Al-Bukhaari, Saheeh Muslim, and other Hadeeth collections. Therefore, Ad-Daaraqutni remarked on some weak reporters, saying, ‘His narration may be taken into consideration’, or ‘it may not be taken into consideration.’”

Ibn Hajar commented on the rationale for citing some weak ahaadeeth as Shawaahid, saying:

“If accepting such weak narrations is conditioned to the presence of similar corroborating narrations reported on the authority of other (authentic) reporters supporting them, someone may ask: what would be the point of the Shawaahid and the mutaaba'at (plural of mutabi', i.e. another hadeeth reported from the same Companion without any change in its wording)? The answer is that the point is to narrate the original hadeeth through different chains of narration and grouping these corroborating narrations together in order to interpret the original hadeeth in light of them all, and in order to underline that this weak reporter was not the only one narrating that hadeeth.” [An-Nakt ʻala Kitaab Ibn As-Salaah]

If through Ijtihaad (personal reasoning), someone reaches the conclusion that a hadeeth is hasan (good), then there is no blame on him if he is qualified to practice Ijtihaad in this field; otherwise, no one may declare a hadeeth to be authentic or weak. Rather, the reference in this regard should be the qualified scholar who is specialized in this field and possesses the required knowledge of its rules.

Allah knows best.