There can be numerous reasons why pages may not appear in the search results or why rankings can drop. One reason is duplicate content. There are many ways in which content can be duplicated and it usually happens unintentionally. It can occur for very valid reasons, often through actions that have been taken to boost rankings. It is not the worst thing to have happen, but if it can be fixed, it is probably in your best interest to do so. I have come across this issue a few times lately and thought I would offer a few helpful tips.
In general, a search engine’s mission is to provide unique and relevant content to the searcher. When an engine comes across duplicate content, the question arises; “Which pages are the most appropriate pages to index?” To display the most useful pages in the search engine results pages (SERPS), a duplicate content filter evaluates, sorts through, and removes the duplicate content pages (and spam). The search engines may do a fairly good job determining what to index, but by taking proactive steps, it is possible to help guide them to the pages you want indexed (or at least keep them from weeding out certain pages from your site). Keep in mind that without providing any guidance, they will do it themselves which may cause disappointment.
Below are just a few ways to avoid duplicate content:
These ideas barely scratch the surface of ways to reduce duplicate content, but hopefully it will get you headed in the right direction.