Well everybody & wordpress too suggests that the perfectly optimised permalink structure should be %category%/%postname%. But today I found a flaw in it. This suggestion can lead you to issues like "duplicate content". Let me demonstrate WHY ??
See this problem is happening because of the category element in the permalink structure. If we are assigning a post under two categories of same order(i.e the categories doesn't share parent-child relation(in case you use multi-level categories)), then this shit is happening.
Assume a post with title "abcdefghijklmnop" as instance. suppose its listed under the categories :
1. Blogging(sub-category of Internet) and
2. SEO(sub-category of Web Design)
So these two categories are of same order.
Now everything seems fine if you are clicking on a link(anywhere) to reach this post. Because everywhere the link to the post is
"http://www.your-blog.com/internet/blogging/abcdefghijklmnop/".(as category "internet" was created earlier than "web design", the default URL that we see uses the "/internet/blogging")
But on closer watch I found that this link(http://www.your-blog.com/web-design/seo/abcdefghijklmnop/) is also shows the same content that is the post "abcdefghijklmnop".
Now I hope you got me. But if you still doubt verify on your own blog(if you are using this permalink structure & have posts under multiple categories of same order). Take such a post & try yourself.
THE REMEDY : If you are already trying this permalink structure then you have to change the permalink structure for the future posts to be safe. And use the Permalink migration plugin to redirect all the old formatted URLs to the new one for the older posts.
And for those who wish to use this permalink structure(bcoz this more search engine optimised) I will suggest to use only one category for a single post.