noun which may take the genitive case particle 'no'
spitting image; exactly alike; like two peas in a pod
Idiomatic expression used almost exclusively for physical resemblance, especially between family members. Often used with ～だ or ～のようだ.
あの双子は瓜二つだ。
Those twins are the spitting image of each other.
彼は父親と瓜二つですね。
He looks exactly like his father, doesn't he?
そっくり is a more general word for 'exactly alike' and can be used for objects, behavior, or situations, while 瓜二つ is limited to physical appearance, especially faces.
Literally 'two melons', from the idea that melons from the same vine look identical. The exact historical origin is uncertain, but the phrase has been used idiomatically for centuries to describe striking resemblance.