Answered by Mufti Mohammed Tosir Miah
Question:
After the fard prayers in my local mosque the imam reads du’as audibly and the congregation says Amins.
He includes the standard du’as which are to be said after prayer, such as “Allahumma anta salam wa minka salam…” etc. Does my sayings of Amin after him suffice for me, or do I have to read these same du’as again for myself?
Answer:
Yes, the Ameen you say after the Imam’s dua would be considered sufficient.
Habib ibn Maslama al-Fihri 
narrates that I heard the Prophet of Allah say: “No group of believers assemble, one of them supplicating while others saying Amîn, except that Allah answers their prayer.” (Imam Tabrani’s Mujumul Kabir & Mustadrak Hakim)
Only Allah Knows Best
Mohammed Tosir Miah
Darul Ifta Birmingham.