You learn SEO best when you get slapped yourself…
To begin, there likely is no Google penalty for duplicate content.
However, when content across pages is similar, Google only displays one of those pages within its search results. This could be a page less relevant than you’d hope for a given user, say a page about a city further away than another you created with fewer backlinks and less authority.
Here’s what happened: for the keyword “Independent SEO Consultant,” I lost my 1st page ranking, then found my LI profile somewhere on the 2nd page.
It seems that when Google identifies duplicate content, it even looks beyond your own site to determine the canonical or primary version for users.
I ended up de-indexing my LI profile yet retaining full visibility within this platform. This seems to have returned that keyword ranking.
Be careful with duplicate content anywhere on the web, not just within your own site. Google may show a page that you don’t want appearing otherwise. I wouldn’t have learned that lesson though without feeling it myself!