Obviously...

Obviously, such a claim is baseless, because many claimed to be the Mahdī in the first era of Islam and in the following ages in which people were still close to the time of the Holy Prophet Allah's blessings be on him and his family, his companions (ṣaḥāba) and the followers (tābi`īn), but we do not see any of the companions or the followers refuting the original concept of Mahdawiyya, but rather, they disputed the claims of the imposters on the basis of their characteristics and features.

Among those subjects that have been narrated to us and we don’t have any method of proving except by hearing them, there is no subject that we must have faith in which is more important than believing in the appearance of the Mahdī, may peace be on him. This is because the glad-tidings that have been narrated concerning him are many and beyond the limit of tawātur. This is while the traditions about most of the beliefs of the Muslims have not reached such a state of tawātur.

In fact, for some of these beliefs, one can only find a single tradition as support and yet, it is regarded as a definite fact. Thus, how can a Muslim—who believes in what the Messenger of Allah, Allah's blessings be on him and his family, has brought and conveyed—have doubts about the reappearance of Mahdī, peace be on him, despite the existence of such abundant traditions?

These traditions cannot be disputed because of the weakness of the chain of narrators in some of them or because of the odd concepts and improbability of the occurrence (of the events mentioned) in others. Surely, the weakness of the chain of narrators does not harm the other traditions which have highly authentic and reliable narrators and contents. Otherwise, we will be forced to put aside all the correct traditions due to the presence of a few weak ones on the subject.

Furthermore, their definitions are well known amongst the faithful and the (religious) leaders of the Muslims, great scholars, and the specialists in the science of traditions have narrated them. Besides, the weakness of the chain of narrators can cast doubt on the narration if the narration is not mutawātir. But, when a narration is mutawātir, the weakness of the chain is no longer a criterion for its reliability.

As for the improbability of the occurrence of some of the events mentioned in the narrations, we can answer: Improbability and oddness have no value in scientific matters—especially those related to the transmission of hadith.