The manual for my 2000 Toy. Solara shows 2 switches in parallel to indicate the driver has buckled his seat belt. Closing either of them will do the trick.
Not only does the manual show the two switches, it gives them separate code letters, like B5 and B6, and mentions both twice in the tables of electrical continuity, etc. So it doesn't seem like a mistake in the manual.
Why do you suppose this switch duplicated? Is it more important than all the other switches in the car?