Should I redirect "ugly" URL to friendly URL?

It creates duplicate content issue and Search Engines dislike duplicate content. Ugly url MUST be validly redirect to seo friendly URLs to avoid duplicate content/pages issue.

You only need to redirect the "ugly" URL if you had changed the URL structure and the "ugly" URLs had already been indexed by search engines and/or linked to by third parties. In this case, it is essential to redirect the old "ugly" URLs in order to preserve SEO and possibly prevent duplicate content issues.

However, if you implemented the "pretty" URLs from the very beginning - and these are the only URLs being referenced, then it's unlikely that redirecting the "ugly" URLs would make any difference in terms of SEO. You should already have the appropriate rel="canonical" tags in-place, which further negates the requirement to implement redirects from the "ugly" URLs.