To the OP, no feeding back the eggshells dose not contribute to them eating their or others hens eggs. We, like many of you, rinse them out, dry them and smash them down. When we get a quart of them we sprinkle them in the garden, where we will plant our tomatoes in a month or two. The birds have access to this when the garden is fallow, so some goes into the ground and some into the chickens.
On the question of calcium in the egg shell, I believe that the calcium in their complete feed ration and that from oyster shell trump egg shells in value to the bird. But we have the shells as a by product, and just as well use them. If you were to be using a feed that did not contain calcium, and also did not offer oyster shells, I don't think the eggs by themselves would be enough to keep you egg structure strong enough for you. By this I am referring to thin shells and even partial or no shell offerings from your hens. Grinding up the shell into a fine powder, the bird is not able to regulate the calcium intake, and can get too much. So I prefer and recommend that they be just crushed and sprinkled on the ground for those whom are so inclined to ingest a little recycled egg shell.
Everyone has their own way and I am always good with that, I just from time to time share my thoughts, and a little on how I came to use them in my flock. Best to you all and your birds,
RJ