Cannot Find Out What Fruit(?) Leaf Is This?

Question

I put some grapefruit seeds into flower pot and it appears that one of them wasn’t grapefruit that came out(?). I cannot find anywhere what plant it might be. In the attached picture one can see a grapefruit sapling to the left (clearly a citrus leaf) but to the right it doesn’t look like a citrus leaf. What is it?

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Answer 1

Pale smartweed (Persicaria lapathifolia) is considered a weed by gardeners.