That's not true. Largest BTC2PLN exchange bitomat.pl lost it's wallet about two years ago. Mtgox bought them with their customers, covered for the mishap and started trading in PLN with very convenient PLN money transfers. In the meantime another exchange was created bitchange.pl but after bitcoin price crash it was closed. It was reborn with same software as pln.bitcurex.pl and now has larger volume of BTC2PLN trade than mtgox. It doesn't take fee on trades though.
So multiple exchanges are possible.