title tag should be present on every page so that when a visitor views your SERP, or bookmarks a page, they'll see a concise description that reflects the contents of it.
As you stated, you should avoid duplicate titles, that is titles which are repetitive or boilerplate. Search engines do allow for branding however, for example:
<title>Stack Exchange (StackExchange) on Twitter</title>
In the above, "Twitter" is the brand, and the unique portion is the username "StackExchange". So using this as an example for the pages containing posts on your site, you can have your brand along with a unique username, or similar as related to the poster or subject.
For more information on Google's suggested title usage, see the section named "Create descriptive page titles" here: Google Webmaster Tools: Site title and description
Also note that you can view problems with your site's titles by following the steps here: Google Webmaster Tools: HTML Improvements
For reasons why to avoid duplicate meta tag descriptions, see the section named "Avoid Duplicate Meta Description Tags" here: Meta Description