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That's an Emerald Swallowtail - Papilio palinurus, native of tropical Asia. So...it had to be an escapee from a nearby butterfly enclosure. The Toronto Zoo has an enclosure and was featuring Malaysian butterflies, including this one. - Frank Smodel
This butterfly is a swallowtail but it is either a strange hybrid (possibly a cross with an admiral?) or an exotic non-North American species.