I was attending a furnal prayer. In that furner prayer Imam Offered five Taqbeer instead of four. In that furner prayer someone gave suggestion to reoffer the prayer correctly and some denied. Alot of confussion was created in that situation. What islam says about this ?
The scholars of the four maddhabs have a consensus that the funeral prayer has four takbeers (or at least four takbeers are necessary to have it performed correctly), even if they may differ on how to pray it. For example the Hanafis and Malikis say it is only supplication (du'a) so one cannot recite Qur'an in it, while the Shafi'i's say it has two tasleems which is based only on analogy (qiyas) and has no support in sunnah.
Case 1: Tasleem before the 4th takbeer
As for the situation where the imam ended the funeral prayer (salat al-Janazah) after only three takbiraat (plural of takbeer) the congregation must tell or inform him, at least by saying "Subhan Allah". But if the Imam didn't act on that one should complete the prayer for himself. The hanabli view is even that this prayer is void if the imam did it inteionally and it is possible to repeat it.
In Sahih al-Bukhari you may find the following narration (see here in sunnah.com where it was not translated as an introduction quote of al-Bukhari in Chapter 64 "There are four Takbir's in the funeral prayers")
وَقَالَ حُمَيْدٌ صَلَّى بِنَا أَنَسٌ رَضِيَ اللَّهُ عَنْهُ فَكَبَّرَ ثَلاَثًا ثُمَّ سَلَّمَ، فَقِيلَ لَهُ فَاسْتَقْبَلَ الْقِبْلَةَ، ثُمَّ كَبَّرَ الرَّابِعَةَ ثُمَّ سَلَّمَ.
Humaid said: "Anas led the funeral prayer and said three takbir and then performed taslim. When he was told about it he faced the qiblah and said the fiourth takbir and performed tasleem (again).
(Source of the translation: see here in Volume 2 page 240)
Ibn Hajar in his Fath al-Bari added another evidence, as he said that the narration of Humaid is disconnected:
Al-Haafiz (may Allah have mercy on him) said: ‘Abd ar-Razzaaq narrated from Ma‘mar, from Qataadah, from Anas, that he said takbeer three times in the funeral prayer, then he ended it by mistake. They said: O Abu Hamzah, you only said takbeer three times. He said: Get back into your rows. So they did so, and he said the fourth takbeer.
End quote from Fath al-Baari, 3/202 (Source of the translation: islamqa #159381 or here in Arabic)
Case 2: What to do if the imam made a 5th or more tasleem?
He also quoted narrations that Anas sometimes only said three takbirs and quoted the discussion on this topic. On which ibn 'Abd al-Barr commented saying that he doesn't know of any scholar (Faqih) saying that the funeral prayer had more than fourth takbirs except with a statement of ibn abu Laila ابن أبي ليلى, who despite being an accepted faqih was a bad hadith narrator (issues with his memorization). See one of the traditions narrated by ibn abi Laila here in Jami' at-Tirmidhi where he explained the view of imam Ahmad and Ishaaq in case of a fifth takbeer.
Note that some reports say that the prophet () performed five or more takbirs on the people of Badr, but these reports are considered as abrogated by the majority of scholars. For example at-Tabarani quoted that 'Ali () made six takbeers on a person who attended Badr this report also appears in al-Mustadrak a-Dhahabi didn't comment on its authenticity as the narrators of at-Tabarani are among the narrators of the two sahhis, but al-Bukhari himself narrated the same tradition without quoting the amount "six" (see here).
In al-Mawsu'a al-Fiqhiya (see point 31 here in Arabic) it is said if the imam said more than four takbirs according to the hanafi madhhab one shouldn't follow as it is abrogated and wait for tasleem in the most sane view or do tasleem after four takbeerat. In the shafi'i madhhab one shouldn't follow and do tasleem after four takbeerat or wait for him according the strongest view (which is also one view of imam Malik in case of an imam who is not from among the people of bid'ah). The lesser recommended view is wait to follow him. In the hanbali view one should follow the imam as long as he didn't exceed seven takbeerat -as some reports of sahabah say that some of them performed up to seven takbeerat- and it is haram to perform salam before the imam even if he exceeded seven takbeerat.
Why don't we perform sujud sahw (prostration of forgetfulness) to repair it?
Finally note that there's a fiqh rule saying:
ما لا يدخل الشيء ركنا لا يدخله جبرانا
Which I will not translate literally but explain (based on this document in Arabic) as the meaning is:
It is essential for repairing (Jabran جبران) a mistake in something essential (Rukn ركن) that the part that repairs it actually is part of it
And as sujud is not part of salat al-Janazah (funeral prayer) one can't repair it by sujud as-sahw. This means either one repeats it or one consider it void or try to correct it (in worst case by oneself).