I mostly have a doubt before the 2nd Sajdah in my Asr & Isha Salat whether it is my 3rd or 4th Rakat, kindly advise whether my Salat is valid or not. If it is not valid the process to make it valid.

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Any doubt before the second sajdah of the second rakat will make the salat batil. If it is after that then one will assume to be 4th rakat in the case of doubt between 3rd and 4th. Finish the salat and then do sajdah sahw.