Prior to the "Black & White" series, when a Pokemon refers to itself by name, interpret that card as though the text reads "this Pokemon". This has the practical effect of not including other Pokemon of the same name; if a Pokemon copies that text, it refers to itself, not the original Pokemon. Beginning with "Black & White", when a Pokemon refers to itself by name, this extends to other Pokemon of the same name; if a Pokemon copies that text, it refers to other Pokemon of the same name. Also, when a Pokemon refers to "this Pokemon", this has the practical effect of not including other Pokemon of the same name; if a Pokemon copies that text it refers to itself, not the original Pokemon.
Source: PUI Rules Team (2008-04-17), TPCi Rules Team (2019-05-16)