That's the superstition of the old people, just like my grandmother wouldn't let me eat fish when I was a child. It is said that eating it will make me stupid and I can't count it. As for what your grandmother said, I don't quite understand. There are customs everywhere.

To know the cause and process of the formation of double yellow eggs:

A double yellow egg is an egg with two yolks in its shell. Double yellow eggs are usually much bigger than ordinary eggs. The reason for the formation of double yellow eggs is that two eggs mature at the same time and are separated from follicles and brought into the fallopian tube, where they are wrapped by protein, shell membrane and eggshell in turn, thus forming double yellow eggs. Sometimes even multiple eggs mature at the same time and are brought into the fallopian tube together to produce multiple yellow eggs. The production of double yellow eggs by poultry is often related to the richness of food and the health of poultry. Double yellow eggs contain two yolks and are very nutritious.