Blog Post URLs are appending ReturnURL parameter when commen

We have a new sitefinity website.  We have blog posts whose urls are being appended with a ReturnURL parameter and this leads to the search engines seeing the same blog post multiple times as different urls. 

Serious SEO issue here and just not clean structurally.

I discovered that this has something to do with the Comments being enabled.  When looking at the page source I see that sitefinity is finding a restricted authentication issue and thus the return url parameter.   See image attached. 

By disabling comments I can avoid this, but ideally we'd have comments enabled.  So please:

1. How to prevent this behavior?

2. Why is this happening?

Thank you - 

Liz

I can't answer to your questions, but i can help you to solve SEO isse.

Just add this inside your robots.txt

That's not an ideal solution b/c it really doesn't solve the underlying problem. 

Can anyone out there (including Sitefinity tech), answer my question about why this is happening and is there anything the can be done to fix it without relying on the robots.txt file or disabling comments?