The Muqtadi (follower) Should Listen and Remain Silent.
However, if a person did recite behind the Imam, Salah will be complete.
Reciting Behind the Imam:
#Allah Ta'aala says: "When the Qur'an is being recited then listen attentively and remain silent so that Rahmah will be showered upon you".
#Hazrat Abdullah Ibn Mas'ud, Abu Hurairah, Abdullah Ibn Abbas and Abdullah Ibn Mughaffal (RA) state that this verse of the Quran was revealed with regards to the Khutba (of Juma'ah) and with regards to Salaah. [Tafsir Ibn Kathir, vol. 1 pg. 281] The dictates of this verse of the Holy Qur'an is that when the Imaam recites the Qur'an aloud, the followers should listen attentively, and when he recites softly, the followers should remain silent.
#Rasulullah (Sall Allahu alaihi wa Aalihi wa Sallim) is reported to have said: "When you begin your congregational prayers, straighten your rows. Thereafter when the Imaam says the takbeer (i.e. when he says Allahu Akbar aloud) you must also say the takbeer. However, when he begins the recital of the Qur'an, you must remain silent. And when he recites walad daul leen then you should say Aameen". By performing your salaah in this manner Allah Ta'ala will love you." [Muslim; ch. on tashahhud] (A similar Hadith has been narrated by Abu Hurairah (radhiallahu anhu)-Imaam Muslim has attested to its authenticity).
-The muqtadi Must Not Recite Surah Fatiha-
#It is reported from Hazrat Ataa Ibn Yasaar (radhiallahu anhu) that he questioned Hazrat Zaid Ibn Thaabit (radhiallahu anhu) concerning reciting Qira'at with the Imaam. Hazrat Zaid (radhiallahu anhu) answered: "There is no recitation of the Glorious Qur'an in any salaah behind the Imaam". [Sahih Muslim, chapter on Sujood-ut-Tilaawah, Hadith 577]
-The Qiraat of the Imaam SUFFICES for the Muqtadi-
#Hazrat Abdullah Ibn Umar (radhiallahu anhu) used to repeatedly say:
"Whoever performs salaah behind the Imaam, the Imaam's qiraat suffices for him". (Sunan Baihaqi; chapter on not reciting qiraat behind the Imaam ? Imaam Baihaqi (R.A.) has stated that this Hadith is Sahih.)
-However, one must recite a surah if a person is performing salah individually-.
#Hazrat Abdullah Ibn Umar (radhiallahu anhu) was asked:
"Must the muqtadi recite behind the Imam?" He replied that the qiraat of the Imaam is sufficient for the muqtadi. But if he performs salaah individually, then he must recite qiraat. It was the practice of Hazrat Abdullah Ibn Umar (radhiallahu anhu) also that he would not recite surah fatiha behind the Imam. (In Aathaarus Sunan (Vol. 1 pg.89) this Hadith has been classified as sahih).
#Hazrat Jaabir (radhiallahu anhu) narrates: that the one who does not recite sura fatiha even in one rakaat, his salaah is not valid. However, if he is behind an Imam he must not recite surah fatiha. (This Hadith has been classified as hasan ? Tirmizi - ch. on not reciting behind the Imaam).
And Allah Ta'aala Knows Best