There’s already a lot of duplicate content on the web. Search engines already have a system to rank duplicate results and not respond with only identical answers. I’m pretty sure this won’t be a big problem. And if it turns out to be a problem, they’ll adjust their algorithms 🙂
Form a technical standpoint lemmy, kbin and co are just as searchable as reddit. Google and co can index them, because everything can be accessed and interpreted using normal web technologies (http, HTML, CSS, …) without needing a login or an invitation to a server and everything is discoverable via links. Perfect for search engines. How good the search results on Google and co are depends on how often and how thorough the search sites will crawl the fediverse. This in turn depends on how important the fediverse sites are deemed by the search engines. This depends on how often they are linked from from the rest of the web and on the amount and qualiry of content and tons of other factors.
Nice, I was trying to figure out how to get that working with Firefox. But, to be fair, it’s not Firefox that’s supporting PWA, it’s the mint webapp-manager which is only included with Mint and requires extra steps to install on other OSes. Not as straight forward as PWA being directly supported by Firefox.