could you please explain to me a quote from a reliable hadith that says "La Mahdi ila Isa" (Ibn Maja) Does it say Isa and Mahdi are same?

You described the hadith as "reliable", while all the great Muhadditheen have classified it as fabricated or extremly weak. From amongst them is: Hafiz Zhahabi (Siwaru A'laamin Nubala Vol.10 Pg.67, Mizaan Vol.6 Pg.132), Ibnul Jawzi (Al-Ilal Vol.2 Pg.862), Bayhaqi (Bayaanu Man Akhta'ah Alash Shaafi'i Pg.297), Nasai (Faidhul Qadeer Vol.1 Pg.363), Ibn Hajr (Tahzheeb Vol.9 Pg.126), Ibn Abdul Bar (Fathul Wahhaab Vol.2 Pg.109), Abdullah Ibn Siddeeq Al-Ghumari (Fathul Wahhaab Vol.2 Pg.108), Ibnul Qayyim (AL-Manaarul Muneef Pg.141-155), Haakim (Al-Mustadrak Vol.4 Pg.488), Saghaani (Al-Fawaa'idul Majmoo'ah Pg.511), Shawkaani (Al-Fawaa'idul Majmoo'ah Pg.511)

It is contrary to the Mutawatir ahadith mentioning the Mahdi (Radhialahu Anhu) to be a seperate entity from Isa (Alayhis Salaam), thus even if it happeened to be sahih, it would not be accepted.

and Allah Ta'ala Knows Best

Mufti Ebrahim Desai