Hi Jorge,
lets take it from the top
Moz tries to show you, and report on how google would see you site.
When you type in a url, the browser and server holding and displaying the website doesn't care if you use capitals or lowercase, for their purpose it is the same page. This is why you will have only created this page once on whatever web platform you are using. However, google sees them differently, each one as a different page.
You could access this page from any combination of capital letters even something stupid like
mysite.com/CoNtAcT-Us.html
These hundred of variations are never picked up on simply because we dont use them.
Lets presume you wanted the the page to be reachable at "mysite.com/contact-us.html" and made it this way. The reason the second variation has been picked up on is most likely because you have used it (or someone else has) to link to that page. Somewhere somebody will have Link Text
Because of this link the second variation is found and because google treats it as a different page, moz is reporting it as a different page.
It is a similiar case with your
mysite.com
mysite.com/index.html
Is is the same page accessible at 2 different urls.
To combat this, you need to use a solution such as
1. Canonical Tags (Recomended)
On your homepage get this code inserted between the tags
On your contact page get this code inserted between the tags
This will cause all versions of this page that are "accidentally made" to say "Hey, im just a copy of this page"
2. 301 Redirects
The second solution is to put a 301 redirect in place, this varies depending on what web platform you are on. This simply redirects the user and any crawl bot to the intented page
i.e. someone tries to go to mysite.com/index.html and your website stops it loading and sends them to mysite.com
This is normally done by editing your htaaccess file. If you want to go this road tell us what platform you website is on and we can give you instructions.