Should one who joined the prayer late repeat the prostration of forgetfulness if he did it with the imam?
Question

                If a person joined the prayer late and the imam had made a mistake and did the prostration of forgetfulness after the salaam or before it, should he do the prostration of forgetfulness again after he completes his prayer? Can the prostration of forgetfulness be done twice?.
Praise be to Allah.
    If the imam makes a mistake and does the prostration of 
    forgetfulness before the salaam, the one who joined the prayer late has to 
    follow him, because he is connected to his imam until he says the salaam. 
    Then when he makes up whatever he missed, he must prostrate again, because 
    his prostration with the imam was not done at the right point in the prayer. 
    The prostration of forgetfulness cannot be done during the prayer, rather 
    his prostrating with the imam was only because he was following the imam. 

    But if the imam’s mistake occurred before the latecomer 
    joined the prayer, then he should not repeat the prostration, because the 
    ruling on his imam’s mistake does not apply to him, as it happened before he 
    joined him in prayer. 

    But if the imam does the prostration of forgetfulness after 
    the salaam, then the latecomer should not prostrate with him, because 
    following the imam in this case is not possible except by saying the salaam 
    with him, and the latecomer cannot do that, as he does not say the salaam 
    until after he has completed his prayer. 

    But if the imam’s mistake came before he joined him, then he 
    does not have to do the prostration of forgetfulness, because the ruling on 
    his imam’s mistake does not apply to him. But if his mistake came after he 
    joined him, then he should prostrate after he has said the salaam.  

    Some of the argument we have mentioned above is based on 
    sound reports, such as the idea that the one who is praying behind the imam 
    should prostrate following his imam, because the Prophet  (peace and 
    blessings of Allaah be upon him) said: “The imam is only appointed to be 
    followed.” Agreed upon. 

    And some of it is based on sound rational thinking, as 
    explained. See: al-Sharh al-Kabeer ‘ala al-Muqni’ and al-Majmoo’ 
    Sharh al-Muhadhdhab. End quote.