Is there gameplay advantage of adding multiple similar parts to your creature in Spore?
At the creature stage (on land) I'm afraid adding duplicate parts doesn't help you out. Essentially your skill level (charm, melee, etc.) is determined by the part with the highest level (for that skill).
I've experimented with this a fair bit, and does appear to be only be available for aesthetic reasons.
There is absolutely no advantage to adding multiple parts.