the Fatwa Department Research Committee - chaired by Sheikh `Abd al-Wahhâb al-Turayrî
Only in a few cases do various weak reports strengthen one another. This is the case when many people narrate the same hadîth from a clearly reliable source. Those who relate the hadîth from that reliable source are all honest and upright, but they might have somewhat weak memories. In this case, since all of them say they heard the same thing from the same reliable narrator, their various narrations corroborate and lend strength to each other.
Such hadîth are called sahîh li-ghayrihî (authentic by virtue of corroboration) or hasan li-ghayrihî (good by virtue of corroboration), depending on the degree of weakness of the mildly weak narrators in question.
In most cases, however, the existence of a large number of chains of transmission for a report do nothing to strengthen that report. For instance, if there are gaps in the chain of transmission, there is the possibility that those gaps could indicate the same weak narrator, or even worse, a liar.
When there are various named narrators in the chains of the hadîth whose identities are unknown, there is the possibility that they very weak or untrustworthy people who quote the same well-known – but inauthentic – text. If they are dishonest people, they might be quoting the same baseless text deliberately. If they are honest but extremely weak, their bad memories might cause them to confuse narrations they had previously heard and attribute baseless hadîth to the wrong narrators.
Therefore, the existence of many narrations of such quality do not lend any strength to the hadîth being reported. This is usually the case.
And Allah knows best.
Source: Islam Today