Duplicate content -- what does that mean? It means that you are using the same text on more than one website. Or that you have "borrowed" content from another website and posted it on yours -- a no no without permission anyway, but now Google might ding you for it. Big time.
It doesn't matter if it's your own content that you have put on two different websites. It doesn't matter if you were flattered to be asked and have given permission for your work to appear verbatim on a colleague's site.
Google doesn't want the confusion of duplicate content. Period.
I could go into all the techie reasons for this, but instead I found this article that explains it pretty well. Click here for the full article on duplicate content.
The bottom line is, just avoid duplicate content. Write something new. It will have a fresher perspective, more up to date facts and sats, more sparkle in the personality of the piece.
You can take inspiration from others' work, and your own previous writings. That's not plagiarism unless you are simply making a bad paraphrasing of the original.
And here's a writing tip -- don't over think it. Just write like you speak. Well, okay, clean it up if you need to. But write as if you are giving advice to a colleague, client or friend. Make it clear and succint, and just stop when you're done.