[wp-hackers] Unique permalinks vs. unique post slugs

[RePost]

Eric Marden wrote:
> Personally, I use categories in my URLs, but only apply one category to a post. This way both the cat and the slug are unique for SEO purposes, and then use tags for all the other topics I want to apply to a post.
>   
Sounds like a reasonable idea, Eric.

Do you then create tags that are identical to each of your categories? 
If so, is there any redundancy factor there, or not?