I have a website with the following url:
But I also have the following url:
Is this really bad for Google SEO ?
If both URLs pull up the same content then you'll have issues with duplicate content. Duplicate content doesn't cause any lowering of rankings but it will force Google to choose one URL over the other to be displayed in the search results as duplicate content will be filtered out.
You should pick one format (I prefer with the trailing slash) and do a 301 redirect with the .html URL. I would also use canonical URLs to make sure Google know which one is the one you wish to have indexed (although if you do a 301 redirect this is unnecessary).
Yes, it does hinder SEO. Search engines will see those as two separate pages and will cause a lower ranking.
You might want to check out this post by ScottGu about SEO, including url rewriting, using the Microsoft SEO Toolkit: Tip/Trick: Fix Common SEO Problems Using the URL Rewrite Extension