I have a website that allows users to find/submit tips for doing specific fitness activities.
The site is structured as "[activity].example.com"
Each subdomain is meant to be a (partially) closed-off community specific to that activity. For example, the swimming subdomain is where all swimming tips are created and shared.
However, some tips appear on all of the subdomains (for example, a "stretching" tip would most likely appear on all activity pages). The top tips may also be aggregated to the root domain. This has been happening quite a bit, and I'm wondering what the SEO implications are.
One example of a site that has a similar setup is reddit. They have their site set up to 301-redirect "[subreddit].reddit.com" to "reddit.com/r/[subreddit]". Presumably this is for SEO purposes.
Am I being hurt SEO-wise by having these subdomains? I.e. is Google seeing them as separate sites with duplicate content? Should I structure the site more like reddit with the sub-directory structure instead?
I know questions related to this one have been answered, but those are more towards exact duplicate content.
Thank you in advance for your help.