I'm wondering if it is prickly lettuce. Can you check for little spines along the midrib on the underside of a leaf, see second picture down in the article link below? There should also be milky sap if you break a leaf off. If you get milky sap but there are no spines under the leaf I might wonder about perennial sow-thistle. If prickly lettuce does not seem to fit can you tell us roughly where the plants are located, just country, state, province will help.
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