Expressions (phrases, clauses, etc.), Noun
(villains) of the same stripe; birds of a feather; badgers of the same hole
Idiomatic expression
一子ちゃんは同じ穴のムジナか、同輩の親しみを込めた笑みを俺に向ける。I guess Ichiko is a fellow rule-breaker, she's sending me a smile filled with affection for a comrade.
Also written 同じ穴の狢【おなじあなのむじな】、同じ穴のむじな【おなじあなのむじな】