Performing more than one umrah when going to Makkah is permissable. Obviously, you'll have performed the first umrah when you go from another country. If you want to perform additional umrahs, you must wear the ihram and go to Masjid Ayesha (about 10-15 mins away from Haram) where you will pray the 2 rakah salah. Then you will perform umrah as normal. However, ulamah recommend that you perform extra tawaf instead of performing so many umrahs as that is the greatest form of ibadah in the Haram.