[Drozh]

Код:

program Program1;

uses
  Forms,
  Windows,
  Unit1 in 'Unit1.pas' {frmClient};

{$R *.res}

var
 MutexHandle : THandle;
const
 MutexName = 'one_inst_demo_mutex';

begin
  MutexHandle := OpenMutex(MUTEX_ALL_ACCESS, false, MutexName);
   if MutexHandle <> 0 then
   begin
    // запуск второй версии
   end
   else
   begin
    // запуск первой версии
    MutexHandle := CreateMutex(nil, false, MutexName);
   end;

  Application.Initialize;
  Application.CreateForm(TfrmClient, frmClient);
  Application.Run;

  CloseHandle(MutexHandle);
end.