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There is a kind of electrical based unique identification tag that is set on a Pokemon that has been captured by a Pokeball. This prevents the Pokemon from being caught by other Pokeballs. The only way this identification tag can be bypassed is if the original Pokeball the Pokemon was caught in is destroyed, or if a Snag Machine is used on the Pokemon. The way a Snag Machine works is it corrupts a Pokeball and charges it with enough electrical energy that it completely breaks and nullifes the electrical identification tag of the original Pokeball.