It is very easy to copy from different websites and claim it your own. It is also very easy to NOT spend time optimizing keywords and other meta information for each page on your website. Although keywords, descriptions and other meta information are said to have no value in your seo rankings, I do know that these engines put duplicate meta information as a bad sign. For example two pages on your site with the same meta keywords will be listed in Google Webmaster Tools as errors needing attention. Take the time to put proper meta information for each page, or develop a programmed system that will do it for you if your website has many pages.
Doorway domains, or domain that masks themselves as your website is very bad for seo as all your content appears duplicated 100%. An example of this would be trying to use different domain names to target different keywords, but all the domains are still your website. If you do this, then stop.
It is very important to pick a www or non-www version of your website and stick to it. http://richlandhosting.com and http://www.richlandhosting.com are seen as two different website and will fall under the duplicate content rule. If your home page is index.php or index.html then be sure to redirect that page. Again for example http://richlandhosting.com and http://richlandhosting.com/index.php is duplicate content. Lastly I would recommend never using trailing slashes. Always print your website as website.com NOT website.com/ unless it is a subpage. The you could use website.com/subpage, but not website.com/subpage/.