Because you can only catch one of each legendary in the games, people believe there is only one of each legendary. For some that might be the case, but legendary pokémon like Lugia, Ho-oh, Articuno, Zapdos, Moltres, Raikou, Suicune, Entei, Latios, Latias, Celebi, Jirachi, Manaphy, etc are not the only of their kind. Some "Legendary" pokémon are only so incredibly rare that they're only ever mentioned in legends.
As for the actual question, i can't even begin to take it seriously.