Duplicate content

Ensure that your pages compliment each other, not compete with each other.

If two pages in your site have identical content, they compete with each other, lowering their rankings and making them harder for your users to find.  

If your domain names and links are improperly managed, search engines can perceive duplicate content where none exists.

Duplication

You may have information that you'd like to have in several pages in your website.  Like this page.  

Not only is it relevant in this section, but it also belongs in Content > Best practice.

To meet this need, but not have the pages compete with each other, use links to direct users to the page in the other section.  This avoids a duiplicate content penalty.

Canonical Duplication

Poorly managed domain name pointing can cause your entire website to compete with itself.

If you have multiple domain names (e.g. yourdomain.co.uk & yourdomain.net), and simply point them all at the same website, search engines will not understand that they are different sites.

Wholething's domain managment service ensures that your secondary domains get your users to your website from all your domain names, but that search engines understand that there you only have one website