Many websites especially e-commerce sites have parts of their site secure using SSL. This protocol allows information to be transferred over an encrypted connection, and is intended to stop packet sniffing devices grabbing your personal details. URLs of secured pages on a site start with the https protocol to indicate to the user that this part of the site is secure.
Some sites will only require the use of https on their contact us page, or in their checkout process to protect the customer’s personal details (bank details etc). On these pages only the protocol needs to be changed from http to https to secure the pages.
This technique however can result in search engines indexing both the secure and non secure version of your pages. If for example you used relative links in your secured pages, this could be interpreted as links to secure versions of your standard pages. This can lead to Google assuming you have duplicate pages, which can result in a drop in your listings due to duplicate content.
So how do you stop Google visiting these duplicate pages?
Go through your pages and replace all your URLs with exact links rather than relative links. You will however need to ensure you link to the appropriate pages with the correct protocol, whether the page is http or https.