asp.net(my record cross together between two user)

It sounds like you are not managing your user sessions properly. It also sounds like you are saving stuff to a text file when you receive the information from the user. You have to virtually "lock" the text file when you receive a request from the user, write their data, and then release the lock again. If you receive a 2nd request while the file is locked, the 2nd user request will wait until the file is unlocked before performing the operation.

// This snippet is incomplete on purpose. How to implement it is yours to figure out. The locking is also not entirely complete.
bool isFileLocked = false;

codehandler( args )
{
   while( isFileLocked )
   {
       // Just wait for 25 milliseconds. Why? Why Not!!!
       System.Threading.Thread.Sleep( 250 );
   }
    if( !isFileLocked )
    {
        isFileLocked = true;
        // Do some stuff.
        isFileLocked = false;
    }
}