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You know, Sean looks insanely similar to Connor.
Literal
Sean [topic-は] Connor [to-に] intensely-similar [copula-なんだぜ].
激似 'extremely similar / dead ringer' is internet/youth slang combining 激 'intense' + 似 'similar.' The 激 prefix is a productive modern intensifier: 激アツ 'super hot,' 激甘 'super sweet,' 激旨 'super delicious.' なんだぜ is the masculine casual assertive ending combining the explanatory んだ with the masculine ぜ.