It certainly is not true that many; if any have double  services in any of the churches I have ever been in....   {That would be a considerable number} Now there are very different beliefs, within the so called Christian churches....  Some have " Jesus Only "  more are Trinity based...  Some follow only " Spiritual Gifts" while other do not recognize such...  Then there is some mixture of both,  with belief, but not any practice...

To get into the Islam belief, one needs to remember, the founder was raised by Christian Parents....  The Islamic teachings are some what twisted, and misleading...  All religions of the world,  except Christanity are based on the works of the individual for salvation...   This is the great divider, and the main difference...  Having a personal mediator in the person of Christ is the rest of the picture....

This is not to say that every so called Christian church and all the members are saved...  Certainly not, but you will find more salvation, and more of GOD's grace in the Christian followers....   That does not mean that thay are perfect, absolutely not, but they have been forgiven....    That is the key to salvation, forgiveness and  the GRACE Of GOD....    
 You can never earn it dear friend, no matter what you or any one else does!!  
Christ paid it all!! It is finished, done, complete... 
 You just have to except it!!    Or not the choice is yours friend...