program Project1;

uses
 Forms,
 Windows,
 Dialogs,
 Messages,
 Unit1 in 'Unit1.pas' {Form1};

{$R *.res}

var
 H: THandle;
 HW: HWND;

begin
 H := CreateMutex(nil, True, 'UniqueName');
 if GetLastError = ERROR_ALREADY_EXISTS then begin

   HW := FindWindow('TForm1',nil);
   if HW <> 0 then begin
     ShowMessage('OK1');
     SendMessage(HW, WM_SYSCOMMAND, SC_RESTORE, 0);
     Exit;
   end else begin

   end;
 end;
 Application.Initialize;
 Application.CreateForm(TForm1, Form1);
 Application.Run;
end.

unit Unit1;

interface

uses
 Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
 Dialogs;

type
 TForm1 = class(TForm)
   procedure FormCreate(Sender: TObject);
 private
   { Private declarations }
 public
   { Public declarations }
   procedure AppMessage(var Msg: TMsg; var Handled: Boolean);
 end;

var
 Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.FormCreate(Sender: TObject);
begin
 Application.OnMessage := AppMessage;
end;

procedure TForm1.AppMessage(var Msg: TMsg; var Handled: Boolean);
begin
 if (Msg.message = WM_SYSCOMMAND) and (Msg.WParam = SC_RESTORE) then begin
   ShowMessage('OK2');
 end;
 
 inherited;
end;

end.

var
   FHandle:  THandle;
   FBuffer:  Pointer;
   FAppHnd:  HWND;

begin
 FHandle := CreateFileMapping($FFFFFFFF, nil, PAGE_READWRITE, 0,
                              4, 'uniquenameformyprogram');
 FBuffer := MapViewOfFile(FHandle, FILE_MAP_WRITE, 0, 0, 4);
 If HWND(FBuffer^) = 0 then
 begin
   HWND(FBuffer^) := Application.Handle;
   Application.Initialize;
   Application.CreateForm(TfrmMain, frmMain);
   Application.Run;
 end
 else
 begin
   FAppHnd := HWND(FBuffer^);
   Windows.SetForegroundWindow(FAppHnd);
 end;
 UnmapViewOfFile(FBuffer);
 CloseHandle(FHandle);
end.