Generally HTTPS and HTTP shouldn't effect your SEO since they are not duplicates since http, and https are just different protocals so to speak. However with this said I have seen my own site with my very own eyes things being indexed as HTTPS and other pages as HTTP. I believe this happens when someone links to the HTTPS and its not yet indexed by Google.
To avoid these situations you should use REL Canonical on your pages, by doing so regardless how people link to it, or if Google crawls it then it will tell Google and other searches engines wait a minute this isn't the MASTER page your viewing and tell Google and co which is.
<link rel="canonical" href="http://www.yourdomain.com/url/to/page" />
There are lots of SEO modules, plugins, addons, and components which will do this for you if your using a open source engine such as Joomla, Drupal, Wordpress and plenty more. A few names are Yoast SEO for Wordpress, and SH404SEF or AceSEF for Joomla.
If you wish to read more benefits about REL canonical I suggest you take a look at Canonical Urls. In this day and age using htaccess to block Google isn't the way to go since you may just end up with other urls and besides.. Isn't people being able to use your site securely if they want to a good thing? HTTPS one day will become the standard and with more and more Content Delivery Networks using HTTPS/HTTP its only a matter of time until everyone is using it. Well maybe in the next 5 years but ya! you get the point. HTTPS is useful and some people use it ;)