To be exactly alike in appearance; used for twins or things/people that look identical (like two peas in a pod).
あの二人は瓜二つで、親戚ですかとよく聞かれる。
Those two look exactly alike; people often ask if they're relatives.
新しいスマホは前のモデルと瓜二つだが、性能は違う。
The new phone looks identical to the previous model, but the performance is different.
子どもの頃の写真を見ると、父と瓜二つだと分かる。
Looking at childhood photos, you can see he's the spitting image of his father.