It means the lemon seed was polyembryonic. That means there were several embryos inside the seed (common in citrus). One embryo is the dominant grower and the others (it could be more than just twins) will die, usually before they emerge from the soil. The littler one is feeling the effects of being out-competed by its more vigorous sibling. The good new is citrus grown from polyembryonic seed will be true to the parent.
If you carefully peel the outer skin of the lemon seed away and look with a magnifying glass, you will be able to see the embryos layered on top of one another.