var
   Semafor     : THandle;

begin
     // Don't start twice ... if already running bring this instance to front
     Semafor := CreateSemaphore(nil,1,'MY_APPLICATION_IS_RUNNING');
     if ((Semafor <> 0) and // application is already running
         (GetLastError = ERROR_ALREADY_EXISTS))
        then begin
             RestoreWindow('TMyApplication');
             CloseHandle(Semafor);
             Halt;
             end;

     Application.CreateForm(....);    
     Application.Initialize;

     Application.Run;
     CloseHandle(Semafor);
end;

EDIT(添加了RestoreWindow方法)：

aFormName是应用程序中您的主表单类的名称。

procedure RestoreWindow(aFormName : string);
var
   Wnd,App : HWND;

begin
     Wnd := FindWindow(PChar(aFormName),nil);
     if (Wnd <> 0)
        then begin // Set Window to foreground
             App := GetWindowLong(Wnd,GWL_HWNDPARENT);
             if IsIconic(App)
                then ShowWindow(App,SW_RESTORE);

             SetForegroundwindow(App);
             end;
end;