Blucube

If your site has an SSL certificate you might find that you can access the site via both SSL (https) and non-SSL (http) URLs. This is a bad idea for security, and for SEO, as it can look like duplicate content on different URLs.
The answer to this is to redirect requests to non-SSL (http) URLs over to their SSL (https) equivalents using something called a 301 redirect. This tells the client (and search engines) that the resource they are looking for should always be accessed over SSL. This plugin offers two methods to achieve this:
<ul>
<li>By intercepting WordPress pages at header time, and if they are not already being requested over HTTPS sending a 301 redirect header, or</li>
<li>By adding mod_rewrite rules in the .htaccess file to redirect all requests to their HTTPS equivalents using 301 redirects.</li>
</ul>
Optionally, this plugin can also set <a href="https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Strict-Transport-Security" rel="nofollow ugc">HSTS</a> headers for you, and make sure that all requests use the same hostname (i.e. fixing the issue where many sites can be accessed using both www. and non-www. URLs).

Simple SSL Redirects

Lightweight plugin to ensure access via SSL/HTTPS. Uses 301 (permanent) redirects for SEO benefits. Optionally sets HSTS and forces canonical domain.

Simple SSL Redirects vs Really Simple Security – Simple and Performant Security (formerly Really Simple SSL)