[Страдалецъ]

Попробуй вот такое решение:

Код:

var
  Form1: TForm1;
  PreviousInstanceWindow: THandle;

implementation

{$R *.dfm}

procedure TForm1.FormCreate(Sender: TObject);
begin
 if WaitForSingleObject(PreviousInstanceWindow, 0) <> 0
 then begin
      ShowWindow(PreviousInstanceWindow, SW_RESTORE);
      Application.Terminate;
      end
 else SetForegroundWindow(PreviousInstanceWindow);
end;

initialization
 PreviousInstanceWindow := CreateMutex(nil, True, 'DJ PhoeniX');
finalization
 CloseHandle(PreviousInstanceWindow);
end.