Indexing categories, can they cause a duplicate content issue in the Google?

Selected answers from the Dumb SEO Questions Facebook & G+ community.

• 
  Gerry White: ... typically categories are good for SEO, tags can create issues, but with xml sitemaps and canonical tags etc you need not worry too much
• 
  Michael Martinez: First of all, "duplicate content issues" are way overrated by most Web marketers. The kinds of sites that have to worry about that problem are HUGE, BIG, AWESOMELY large. Secondly, on a blog, categories and secondary index pages only create "duplicate" content if you publish full posts instead of excerpts and/or double index posts in the same pattern (assign them to multiple categories or tags).
• 
  Casey Markee: As an example, on Wordpress, you can optimize the CRAP out of Category pages. These can in turn lead to great amounts of traffic. Here`s an example: https://champagne-tastes.com/category/bourbon/ See how much more content-rich this page is than a usual category page? See the custom header with actual internal links to related content? Further, this page is fully optimized with a customized Title, Meta Description and on-page H1. If you do this for all your category pages it can generate a substantial amount of traffic on a monthly basis. Tag pages....not so much. Whenever you can optimize Category Pages you should. That`s just smart SEO.
• 
  Zubair Ahmed: Hi Azeem Iqbal, first of all, think why would you want to get your category pages indexed? Your category page, as it may be now will just be a collection of all your posts under a particular category and nothing else. They may be showing the entire post or an excerpt of your post depending on your theme. You try to rank a page based on a keyword, but what keyword will you try to rank your category page for? It is ok to get your categories indexed if you have a unique description displayed in your category page for each of the categories and are trying to optimize and rank for that page. Better to leave them unindexed unless you explicitly focus is to get these category pages to rank for something else it`s not worth the time and effort.
• 
  Roger Montti: >>>why would you want to get your category pages indexed? There is a very good reason. But... I`m not sharing today.
• 
  Stockbridge Truslow: You want category pages indexed in almost every scenario.
• 
  Neil Cheesman: Personally, I would say it depends on the site architecture - but IF you can get a category page to rank on Page 1 of Google (and not to the detriment of any other `page`) then why not?
• 
  Doc Sheldon: As Stockbridge Truslow says, allowing indexation of category pages is nearly always the right path. And if you think about it from the aspect of an ecom site, particularly, transactional queries are more likely to be constructed more like a category than a specific product (for example, "digital DSLR cameras" versus "Nikon D3200 camera")