[Kipow]

Este codigo me sirvio a mi:

Código Delphi [-]
var
   Mutex : THandle;

begin
   Mutex := CreateMutex(nil, True, 'miEXE);
   if (Mutex <> 0) and (GetLastError = 0) then
   begin
      Application.MainFormOnTaskbar := True;
      Application.Initialize;
      Application.Run;
      if Mutex <> 0 then
          CloseHandle(Mutex);
   end;
end.