My proposal would be to offer some flexibility, so there is a choice of what type of community to make without getting dinged by Google for duplicate content:
- Give members of a blog that uses Discourse the full privilege of reading and responding to an article on Discourse.
- Make Discourse the ‘clubhouse’ that only shows an excerpt of the article. This does not require a canonical url set to the blogpost.
The first option would require a canonical url pointing to the blogpost that is not on Discourse. The easiest way to offer this flexibility imho is an option to set the canonical url for a topic.