I know that we recite bismillah two time in surah namal but i want to know that what is the reason behind reciting second time.
The other Bismillah is part of the narrative of the Surah. Suleman عليه السلام is informed of the Kingdom of Saba (verses 22-26) and he sends a letter to their Queen (verse 28) which reads:
إنه من سليمان وإنه بسم الله الرحمن الرحيم
ألا تعلوا علي وأتوني مسلمين
Indeed, it is from Solomon, and indeed, it reads: 'In the name of Allah, the Entirely Merciful, the Especially Merciful,
Be not haughty with me but come to me in submission [as Muslims].'
Basically the letter begins with Bismillah, and we are repeating the contents of the letter.
The answer is Solomen sent an invatation to a Queen That letter starts in bismillah