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.
bool isFileLocked = false;
codehandler( args )
{
while( isFileLocked )
{
System.Threading.Thread.Sleep( 250 );
}
if( !isFileLocked )
{
isFileLocked = true;
isFileLocked = false;
}
}