before starting every work as a muslims we say "bismillah". before starting namaz in ist rakat we say bismillah is it important to say bismillah in 2nd rakat and 3rd rakat or not?
If you are reciting al-Fatihah in the first rak'a starting with bismillah following the Kufi qira'at and Moshaf (For example qira'at Hafs 'an 'Asim the mostly known qira'a nowadays) you should recite it in the second too as it is part of the qur'an according their opinion.
Else in the Hijazi Moshafs and qira'aat (The Madani for example) it is not part of al-Fatihah nor at the beginning of any other surah, except with the verse in the middle of surat an-Naml (where it was quoted in the middle of the surah and therefore there's a clear consensus that it is part of the qur'an) but a blessing one should say when reciting qur'an. So it can be left out according to this view. But many scholars hold the opinion that avoiding disputes is better and one could say this blessing for oneself.