The mehdi of shia and sunni are entirely different
The 12th Imam of the Shia is Dajjal
* Introduction
The Shia are waiting for their Twelvth Imam to descend upon them. The evidence suggests that the person that the Shia will take as their Twelvth Imam will be none other than Dajjal, as prophecized in the Hadith of the Prophet. The Ahlus Sunnah should thus always keep in mind that the Shia are the future followers of Dajjal.
* Jewish Law
The “Qaem” or Imam Mahdi of the Shia will rule according to the Laws of David and the family of David, according to the Shia Hadith in Al-Kafi, the most reliable of the four books of Shia Hadith. All of the following Shia Hadith come from the same section in Al-Kafi reported by Al-Kulyani, located in Al-Kafi (Volume No.1, p.387-398):
Narrated Ali ibn Ibraheem -from his father-from ibn Abi Umair-from Mansour-from Fadhl al Aour-from Abi Ubaidah who reported:
“When the Qaem of the household of the Prophet appears, he will rule according to rule of David and Solomon.”
Muhammad ibn Yahya-Ahmad ibn Muhammad-Muhammad ibn Sinaan-Abaan who reported: “I heard Abu Abdullah[as] saying: ‘The world will not fade away unless a person from us appears who will rule according to the rule of the family of David’”
Ahmad–Ahmad ibn Muhammad–Ibn Mahboob–Hisham ibn Salim–Ammar as Saabati who reported: ‘On what will you rule if you are made the rulers.’ He replied: ‘By the rule of God and the rule of David. And if we are confronted by a situation which we cannot solve, Gabriel (Ruh al Quds) will reveal it to us.’”
Muhammad ibn Ahmad–Muhammad ibn Khalid–Nazr ibn Suwaid–Yahya al Halabi–Imran ibn Oueiyn–Jaeed al Hamdani–Ali ibn al-Hussein [as] said: “I asked him by which law will you rule?” He said: “By the rule of David, and if there is something which we are unaware of, Gabriel (Ruh al Quds) will reveal it to us.”
Ahmad ibn Mahran[ra]–Muhammad ibn Ali–Ibn Mahboob–Hishaam bin Salim–Ammar as Saabati reported: “I asked Imam Abu Abdullah [as]: ‘By what will you rule?’ He replied: ‘By the rule of God, and by the rule of David.’”
* Hebrew
Furthermore, the 12th Imam will speak in Hebrew:
Reported to us Ahmad ibn Muhammad ibn Saeed al Uqdah who said: Narrated to us Ali ibn al-Hasan at-Taymali who said: narrated to us al-Hasan and Muhammad the sons of Ali ibnu Yusuf, from Sa’daan ibnu Muslim, from rajaal, from al-Mufadhaal ibn Umar who said: Abu Abdullah [as] reported: “When the Imam Mahdi calls out, he will supplicate to God in Hebrew.” [Al-Ghaybaa of an Numani, p.326]