Duplicate index.html

so I have made an HTML, CSS, and js site and when I checked for SEO errors it gave me that my website has two duplicate titles. Hosted in Cpanel.

My landing page is tescooffice.lk and the other page is tescooffice.lk/index.html

Both of these pages are on the same page but have different URLs. How do I change this for only one URL, not two? Feel free to give me any corrections to my website.