I can"t respectfully disagree on your assessment of the source of the malformities. The four-legged chick was from a farm close to us. It didn"t actually hatch out, but was discovered when their son opened up all the unhatched eggs (as he did every time they set eggs).
That young lad was also the source of information about the two-headed chick, which he described as actually more like having an extra beak growing out of the head. That one I didn"t get to see because he said it was too gross for him to handle. He blamed both on setting double-yolked eggs, but that information might have been his after-the-fact observation.
As said previously, we were not allowed knowingly to use double-yolked eggs for setting and the double and triple-yolked eggs we occasionally would get were so large that it was obvious they were not normal eggs.