concentration

I would like to first say that this  website is very  helpful, may Allah bless you all for the service you provide.  My question regards  the fardh prayer, I know you are supposed to have full  concentration during it, but I have heard that once you perform the prayer you are not supposed to repeat it.  I want to know whether it is ok to repeat it if you feel as though you have not concentrated properly or your mind swayed to other thoughts during the pray and you are generally not satisfied with the prayer. 

Dear Respected scholarassalam Alaikum wa rahmato AllahI have trouble keeping my concentration during salat.  Several bad words and kufr comes in my brain while I am praying and sometimes I have to stop my salat.  I am fighting hard to keep my salat clean from that kufr but I have not been able to do that.  Everytime I start praying wether at home or at masjid the same thing happen. Please advise me about what to do and how to avoid that.  I know this is from shatan but what do I do to stop it?also will my salat be accepted despite all these bad ideas that shatan bring into my prayer?  Also can some stop salat if this happens?ThanksJazakum Allah

dear mufti saab sometime in salat i doubt that i made intention for that salat during or after the salat or sometime right after i wudu i doubt did i still have my wudu or even thought i did not

As-Salamu `alaykum. When we pray we must think of Allah. How do we get into this frame of mind and keep our attention concentrated on Allah?