Most SEO’s will know that having a http and https version of your website is a really bad idea, especially when the content on both sites are exactly the same. However duplicate content issues are not the only reason that this is a bad idea as I found out recently.
I was working with a client who had both http and https versions of their site live for legacy reasons. I had recommended that the redirect the https version of the site to the http version, however because the https version had a rel canonical tag pointing to the http version, my recommended redirection was not a priority and was pushed to the backlog.
When the client wanted to launch a new market, I was helping the client install the Google analytics tags on the page. While installing the tags, I noticed the client had a huge drop in traffic at a point when the site was redesigned. Quizzing them about it, I found out they figured that they lost the traffic due to their redesign. Closer investigation revealed that after the redesign, they had not lost any of their previous rankings and the traffic only seemed to be dropping on their homepage. This seemed weird as you will generally equate a loss in traffic to a loss in rankings and you will expect the drop to affect the entire site and not just a page.
I decided to look into the implementation of the Google analytics tags on their page and everything seemed fine, on a hunch, I decided to check the https version of the site and voila… They were using a Google analytics tag with the wrong account number on the https version of the site. Meaning all the traffic to the https site was being attributed to some random persons account (if that person was into advertising, they would have been very ecstatic).
The problem has now been fixed and the client has seen their traffic levels return to their normals levels plus now the redirection from https to http has become a priority. Guess every cloud… 
