The phenomenon is known as "ovum in ovo"
A woman from Somerset discovered another egg inside the boiled egg she was preparing to eat for her breakfast.
Ann Lewis, 47, from Taunton, said she had eaten plenty of "double-yolkers" before but that she was very surprised at the unusual find.
Douglas Russell, from the Natural History Museum, said: "A complete egg found within a complete egg is relatively rare."
Ms Lewis, who works as a waitress, bought the egg from a local farm.
Douglas Russell, speaking about the phenomenon in the New Scientist, said: "As the curator of the British Natural History Museum egg collection, I've come across quite a few examples of egg oddities.
"Double eggs (as opposed to multiple-yolked eggs) are less common than some other zoological anomalies and consequently the ovum in ovo has attracted specific scholarly attention for hundreds of years.
"Several theories have been proposed for the origin of double eggs.
"The most likely suggests that the normal rhythmic muscular action, or peristalsis, that moves a developing egg down the oviduct malfunctions in some way."