Doppelganger Eggs are an Alchemical product. They are only a bit less expensive than a Philosopher's Stone and require extensive research as well as rare and expensive materials. This is the sort of secret one could obtain by dealing with demons and the like.
The Egg will hatch a clone of the last creature to touch it. It takes 3 hours for the egg to hatch after being touched. The hatchling will be the same size as freshly born or hatched creatures normally are for the species in question. Prematurely cracking the egg destroys it and yields nothing but egg white and yoke. No signs of what it could have been are visible. The new being will grow at a rate normal for its species.
There is a high probability that the being emerging from the egg will 'imprint' on the next being it sees, with the exception being the True Doppelganger Egg below.
Greater Doppelganger Egg:
As normal, but a full adult being emerges from the egg - growing to full size rapidly. In this case, the clone has most of the original being’s memories, but its personality will be randomized.
True Doppelganger Egg:
As a Greater doppelganger egg, but the memories and personality are identical. They are still distinct beings and do not share a soul or consciousness.
The New King
The Kings enemies are plotting to replace the king with a magically dominated clone. The PCs could interfere with the attempts to obtain the components or ritual, or they may be involved after the Clone has replaced the true King.
Another Big Baddie.
In the land of Somesuch, there is a vast and powerful beast that has been magically bound with unbreakable fetters. The Villain wants it, but instead of the herculean task of breaking it free, he simply seeks to clone it with a Doppleganger egg. Again the PCs could either prevent this, or deal with the aftermath.
We Need One
For whatever reason, the PCs get roped into searching out the rare components for the egg, and perhaps helping guard the alchemist during its manufacture. Perhaps someone of power needs a new host body - the close is fully sentient, so this would pose potential ethical challenges.
A New God
Someone, someone managed to get a demigod to touch the egg unawares, and now there is a cloned God in existence. What type of havok will this bring?
I leave it up to the individual GM as to whether the subject touching the egg must be alive for this to function. In the case where dead beings can be cloned, well, this opens up a lot of potential and abuse... You may in fact like that.