I have a SEO Problem. Can anyone clear this for me?

I have a wordpress blog up and running. But now I want to make another one. I have selected some keywords to target but the thing is, I don't know whether to target them to pages or posts.

Like, I can target my keywords to separate pages or separate posts. But in terms of SEO what is better?
Is the category-pages going to be regarded "duplicate content" as they are under two urls? (the main blog post url and the url under category).

Now search engine robots consider on html page to be on page. So what happens to he blog's main page where there are a lot of posts? Like each post is targeted to a different key word. So interms of SEO the main page will be ineffective right?

Can anyone clear these problems?