Now if I write an application, e.g. abc.exe, the user could run more then
one copy of the program when they click it in the explorer many times. So how
could I do if I want the user can only run only one copy of that program, i.e. if
the program (abc.exe) already started and the user double click its icon in the
explorer, then the abc.exe will got focus (not a new process).

Thank!