In what ways does Gollum act as a doppleganger or foil for Bilbo Baggins?

They are both hobbits (Gollum was at the least closely related); they are both used by the ring, gain an agelessness through the ring, and give up a large part of themselves to an obsession over the ring. These similarities also bring out their differences. Bilbo has a much better understanding of the ring than Gollum. Bilbo is able to resist the ring's power while Gollum is consumed by it.