So it means,
Point one: Of course you should not submit same content to blogs you are guest blogging as it could harm them and ultimately your efforts would also lost.
Point 2: Your blog content need to be strictly unique as they are in your domain, you should not use that kind of content for other purpose.
What do you think about article submissions. Some agencies i know say its safe to submit one article to 25 -30 article sites and it helps in SEO.What do you suggest?