Avoid duplicate URL?

Hi,

One of my site is using nginx server and every time we created a content two URLs are generating as below.

http://example.com/topic-how-to-avoid-duplicate-url.html
http://example.com/Topic-How-To-Avoid-Duplicate-Url.html

Our preferred version in http://example.com/Topic-How-To-Avoid-Duplicate-Url.html but Google is indexing http://example.com/topic-how-to-avoid-duplicate-url.html (completely small case URL)

Really worried as it is creating duplicate content. Like this I am having 500+ pages.

Looking for your suggestions on whether I need to change any settings are place any code to redirect http://example.com/topic-how-to-avoid-duplicate-url.html to http://example.com/Topic-How-To-Avoid-Duplicate-Url.html

Thanks for looking on to it.