Guest posting

If it's already on your site, it won't be 'unique' to the site you post it on next, will it?

Not at all. You're apparently confusing "duplicate content" with "syndicated content", as many people do. This thread may help you.

Yes, if the owner of the blog will accept it.

Some blog-owners will have the idea, in doing a "guest-post" deal with you, that they're getting previously unpublished content. It's "negotiable".

When I do this (and I don't often do it, I admit - I generally prefer blog commenting to guest-posting), I'll only give them content previously published on my own site, because to me it's just "syndication" and I'm not letting anyone else have initial indexation rights to my work, whether it's a blog, an article directory, or whatever. If I found a compelling, high-traffic, valuable, highly relevant blog offering me a high-PR backlink (not just a blog with a high-PR home page, of course), I'd happily change my mind about that, though.

This can sometimes be a slightly moot point when you send articles to people for syndication, as well. One can offer them articles "not yet submitted to any article directory" which have nevertheless previously been published on one's own site, provided they're not going to feel "misled" by that. One has to judge the exact form of words according to how well one knows them, and so on. Quite apart from the "morality" of such situations, though, it's certainly not worth creating any ill-will over it: the long-term value of a relationship with someone who'll syndicate content to a relevant site is worth far more than that.

All of the websites I've guest posted on (and there's been a few!) have wanted original content. Your mileage may vary, but it'd be a good idea to ask the website owner what they want.

If you are an expert in the field that the person wants the guest post on, let them interview you instead. They get content that's almost completely written for them - you get noticed as an expert. Win/Win.