Definition #1 (http://dictionary.reference.com/browse/ego):
the “I” or self of any person; a person as thinking, feeling, andwilling, and distinguishing itself from the selves of others and from objects of its thought.
This definition illustrates that the Ego is inherently associated with the distinction between “I” and “everything else,” thus creating a duality. When the ego dissolves, the concept of “I” as an individual ceases to exist.
The ego dissolves in stages, at each next step inducing a greater sense of Unity.