From my experience, this problem is most common on websites that use .NET. The problem stems from the fact that the server is configured to respond to URLs with uppercase letters and not to redirect or rewrite to the lowercase version.

I will admit that recently, this problem hasn't been as common as it was because generally, the search engines have gotten much better at choosing the canonical version and ignoring the duplicates. However, I've seen too many instances of search engines not always doing this properly, which means that you should make it explicit and not rely on the search engines to figure it out for themselves.

How to solve:

There is a URL rewrite module which can help solve this problem on IIS 7 servers. The tool has a nice option within the interface that allows you to enforce lowercase URLs. If you do this, a rule will be added to the web config file which will solve the problem.

Search Engine Optimization (SEO) is a very general term and a very dynamic industry. Every so often the SEO paradigm that we have come to know changes and nobody really knows for sure how things change.