I actually just did a wealth of research on this topic a few days ago. Without going into the nitty gritty details, if the https is site-wide Google recommends a Rel="canonical" attribute (http://support.google.com/webmasters/bin/answer.py?hl=en&answer=139394) pointing to the non-secure http version. Google claims it can index https fine, but Matt Cutts said he would "lean towards pointing the canonical to the http version." Also, on the Rel="canonical" page Google says:
If you publish content on both http://www.example.com/product.php?item=swedish-fish and https://www.example.com/product.php?item=swedish-fish, you can specify the canonical version of the page. Create the element:
Add this link to the section of https://www.example.com/product.php?item=swedish-fish.
Make sure the canonical is on every page of your site.
Not sure why this may have happened, but it is creating duplicate content, which is why the canonical is necessary.
Hope that helps!
Thanks