Protect your own copyrightIf you wrote it, then it is your copyright unless you sign that copyright away. Always read the small print of any site you want to publish your article on.
Be clear of the conditions under which you are submitting your content to any site and check whether people are complaining about the site using articles they have deleted.
There are unscrupulous sites that will make money out of your work whether you are still on that site or not.
Face book users should always put in that their work is their copyright with anything they post. Personally, I have stopped publishing anything I write directly to facebook and a lot of people put a notice on their page to say they retain copyright of their work.
Copyyright is NOT what is meant by Duplicate.Whatever you put on the web is published and may show up as such even if you just placed it somewhere as a draft. Once it gets on the web, then it is published. Keep your work on your own hard drive for that reason alone until you are sure of where you wish to place it.
Once your article is on the web, then any further copies of it wherever you put them are seen as duplicate copies even thought they are still your copyright.
Search engines do not like duplicates. Google has clear web master tools that explain why they do not like this. They are worth reading.