Complex sentence: Contributors may create their own topic, or post an article to an existing topic (which may have been created by another contributor, or by a Helium staff member), but each writer can only submit one article to any one topic.
Split into simpler sentences: Contributors may create their own topic, or post an article to an existing topic (which may have been created by another contributor, or by a Helium staff member). Each writer can only submit one article to any one topic.