Как не допустить запуск второй копии программы 2

program Previns;
uses
  WinTypes,
  WinProcs,
  SysUtils,
  Forms,
  Uprevins in 'UPREVINS.PAS' {Form1};
{$R *.RES}

type
  PHWND = ^HWND;

function EnumFunc(Wnd: HWND; TargetWindow: PHWND): bool; export;
var
  ClassName: array[0..30] of char;
begin
  Result := true;
  if GetWindowWord(Wnd, GWW_HINSTANCE) = hPrevInst then
  begin
    GetClassName(Wnd, ClassName, 30);
    if StrIComp(ClassName, 'TApplication') = 0 then
    begin
      TargetWindow^ := Wnd;
      Result := false;
    end;
  end;
end;

procedure GotoPreviousInstance;
var
  PrevInstWnd: HWND;
begin
  PrevInstWnd := 0;
  EnumWindows(@EnumFunc, Longint(@PrevInstWnd));
  if PrevInstWnd <> 0 then
    if IsIconic(PrevInstWnd) then
      ShowWindow(PrevInstWnd, SW_RESTORE)
    else
      BringWindowToTop(PrevInstWnd);
end;

begin
  if hPrevInst <> 0 then
    GotoPreviousInstance
  else
  begin
    Application.CreateForm(TForm1, Form1);
    Application.Run;
  end;
end.