About a month back I planted some dahlia and marigold seeds in a pot. It seemed unsuccessful till this sprout came up.
However, I'm not sure if it's actually something I grew or worth keeping.
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Seedlings are difficult to identify, but that certainly could be a dahlia. At least, I don't see anything that would rule it out. The general shape of the leaf is right, and so are the teeth along the edge. If it is a dahlia, when it grows new leaves, the leaf veins should become more indented on the newer foliage.
Here you can see some dahlia seedlings:
Compare them to the foliage of this mature dahlia plant:
Notice that the mature leaves have more serrations, and the texture of the leaf is less spongy-looking and more thin, glossy and with slightly indented veins. Also, the mature leaves are compound.
I would suggest letting your seedling grow, and watch for it to develop normal dahlia leaf characteristics. As long as it does, I would assume it's a dahlia until it blooms and proves the case one way or the other.