Not including battle frontiers which legendary Pokémon (including Mythical Pokémon but not ultra beasts) show up more than once in their home regions in the video games? (ex: npc having one, can catch more than once, etc)
In some cases, the Pokédex mentions multiple Legendary Pokémon of the same species. The Pokédex says about Latias: "They make a small herd of only several members." The Pokédex says about Entei: "It is said that one is born every time a new volcano appears." The Pokédex says about Cobalion: "From the moment it's born, this Pokémon radiates the air of a leader." implying its birth is a repetitive occurrence. The Pokédex says about Urshifu: "Inhabiting the mountains of a distant region, this Pokémon races across sheer cliffs, training its legs and refining its moves." as Urshifu evolves from Kubfu, this implies that wild Urshifu exists elsewhere.
Sometimes, multiple Legendary Pokémon are seen in the same game. In Pokémon Red and Blue, as well as in their remakes, the player can see a wild Articuno at the Route 15 gate to Fuchsia City, through a pair of binoculars at any time, even after catching Articuno at the Seafoam Islands. In Pokémon: Let's Go, Pikachu! and Let's Go, Eevee! the Legendary birds are rare spawns, allowing the player to encounter and catch multiple of each in a single save file. Because Type: Null and Silvally are synthetic species, it was possible for humans to create multiple individuals. In The Isle of Armor, the player receives a Kubfu, while Mustard also has one. Mustard's Kubfu will evolve into an Urshifu of the form corresponding to the tower not chosen by the player. In The Crown Tundra, both the Kantonian and Galarian regional forms of the Legendary birds can be encountered in Crown Tundra. Most Legendary Pokémon can be encountered multiple times in Dynamax Adventures in the Crown Tundra. However, the player can only catch one of each.
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