Siamese Twin Plant ?

Quite a common occurrence/mutation with cannabis and not really anything wierd, or something that will affect growth, yield or potency. Does look like a mag deficiency too, which will appear across the entire plant all at once and not just in the older leaves first. Deficiencies of macro nutrients (N/P/K) generally show in the older leaves first as they are "mobile" nutrients, meaning the plant will move/take them out of the older leaves to use in the new growth areas, and minor nutrients/trace elements (such as magnesium, iron, copper, sulfur etc) are "immobile" nutrients, meaning their deficiencies show up in the new leaves first because the plant can't access them from the soil or from the older leaves for use in the new growth areas.

Just here cause I love weird mutations. You might have a sulfur deficiency because the top ones are light green with dark green veins. @Roberts, I'm sure you're much more of an expert but wouldn't a magnesium deficiency appear on bottom leaves first?