It will happen under certain conditions.
This is also how you can duplicate items on a server.

Basically the world save is off from server reset time.

Server resets and world goes back to its last save.

So you can wait for after the save pull crops or stuff from boxes, log out, then log back in after reset and stuff will still be in the ground and in boxes and also duped in your inventory.

So you probably entered the crypt just after the world save, when the server reset it reset to the point it had saved the world and you duped the iron,