Google Webmaster Tools Index Status Issues

I have a site that, according to Google Webmaster Tools index status, has twice as many indexed URL's as there are html pages on the site (according to a screaming frog crawl). I can't find any duplicates either. Does anyone know how this can happen or point me in the right direction to look?

You can try to find out what kind of "extra" content is being indexed in Google by using the site: operator without filter.

1. Search for this:

2. Repeat the search adding the following parameter to the Google SERP result URL:

Then compare both results. On the last SERP generated by the second step you will see a sample of results from your site that Google consider to be duplicate content and did not showed up on the first step search. May be there are some URLs with parameters that Screaming Frog couldn't catch.