If there is an existing patent the holder will have exclusive rights to the manufacturing, distribution and sales within the country where the patent is registered until the end of the patent. But if the holder of the patent has been unable to exploit the patent within a specific time frame (I can't remember exactly for how long) anyone can file a petition to exploit that product and pay a royalty to the holder. This is commonly seen in products that have their patents registered in several countries but that are unable to exploit that specific product in all of the countries where it has been registered.