I am running Free Article Posting site but I discover people are posting their content which already has been published on different sites before. What should i need to do in order
to save my site from Google penalty. waiting for your kind help in this regard
It’s a matter of semantics. While there is not a specific penalty named “duplicate content penalty”, having significantly identical content can hurt you–which is basically a “penalty”. From Google:
Google tries hard to index and show pages with distinct information. This filtering means, for instance, that if your site has a “regular” and “printer” version of each article, and neither of these is blocked with a noindex meta tag, we’ll choose one of them to list. In the rare cases in which Google perceives that duplicate content may be shown with intent to manipulate our rankings and deceive our users, we’ll also make appropriate adjustments in the indexing and ranking of the sites involved. As a result, the ranking of the site may suffer, or the site might be removed entirely from the Google index, in which case it will no longer appear in search results.
So having content which has been published elsewhere can lower your rankings, have the page de-indexed, or get your site blocked from further indexing. Those are penalties–regardless of the words you use to describe them.
To the OP: There’s not much you can do from a technical standpoint to block duplicate content. There are several things, however, that you can do from an administrative standpoint.
Make it clear in your TOS that the site only accepts original material that has not been published elsewhere.
Reward those who contribute original, unpublished works (special titles, higher “rank”, etc.)
Penalize those who contribute previously-published works (reduced “rank”, eventually banning).
Offer a “bounty” to anyone who presents verifiable evidence that someone is contributing previously-published works (risky, as it could cause in-fighting).
Combine this with other SEO best-practices, and you shouldn’t have any issues.