Prevent Multiple Instances of Delphi Application Running

This code will prevent multiple instances of your application from opening, and pass parameter details to the existing application process.

var WM_FINDINSTANCE: Integer;

{The basic code contained in the next three procedures,
 which works with the DPR code to allow file association
 and open double-clicked file in the running instance of the app
 was written by Andrius Adamonis}

procedure TForm1.WMRestoreApp(var Msg: TMessage);
begin
  if IsIconic(Application.Handle) then
    Application.RESTORE
  else
    Application.BringToFront;
end;

procedure TForm1.DefaultHandler(var message);
var
  s: String;
begin
  with TMessage(message) do
    if (Msg = WM_FINDINSTANCE) then
    begin
      Result := MyUniqueConst;
    end
    else
    if Msg = WM_OPENEDITOR then
    begin
      SetLength(S, MAX_PATH);
      GlobalGetAtomName(WPARAM, PChar(S), MAX_PATH);
      ShowMessage(S);
      try
        OpenDocument(S);

        except on E: Exception do
            raise Exception.Create('Error opening ' + S + ' - ' + E.message);
      end;
    end
    else
      inherited DefaultHandler(message);
end;
This code goes at the very end of the main unit

initialization {Used in the file association and running instance code}
  WM_FINDINSTANCE := RegisterWindowMessage('Your Custom Handle Message Here');

  if WM_FINDINSTANCE = 0 then
    raise Exception.Create('Initialization failed');

end.

program Sample;

uses
  Forms,
  SysUtils,
  graphics,
  Windows,
  Unit1 in 'Unit1.pas' {Form1},

{$R *.RES}

var Previous: HWND;
    ATOM: TAtom;

function EnumWindowsCallback(Handle: HWND; Param: LPARAM): Boolean; stdcall;
  function IsMyClass: Boolean;
  var
    ClassName    : array[0..30] of Char;
  begin
    GetClassName(Handle, ClassName, 30);
    [i]/If you change the name of Form1 don't forget to change the string below![/i]
    Result := (StrIComp(ClassName, 'TForm1') = 0) and (SendMessage(Handle, WM_FINDINSTANCE, 0, 0) = MyUniqueConst);
  end;
begin
  Result := not IsMyClass; [i]{ needs True to continue }[/i]
  if not Result [i]{ = MyClass }[/i] then Previous := Handle;
end;

begin
  Previous := 0;
  EnumWindows(@EnumWindowsCallback, 0);

  if Previous <> 0 then
  begin
    PostMessage(Previous, WM_RESTOREAPP, 0, 0);
    if (ParamCount > 0) then
    begin
      ATOM := GlobalAddAtom(PChar(ParamStr(1)));
      SendMessage(Previous, WM_OPENEDITOR, ATOM, 0);
      GlobalDeleteAtom(ATOM);
    end;
    Exit;
  end;

  Application.Initialize;
  Application.Title := 'Sample 1';
  Application.CreateForm(TForm1, Form1);
  Application.Run;
end.