Unique BGP Regular Communities

Hi, When the providers choose communities do they follow the syntax AS_NUM:X, where X is some number to ensure uniqueness of their particular community? The reason i ask this is because if operators are doing this then they need not worry that the community being used by them would not be used by anybody anywhere in the world. I am wondering if it can _ever_ happen that i get to recieve a BGP UPDATE carrying a community number that i use inside my AS? Is this possible? And if Yes, then what scenario? Thanks, John