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It's tricky, honestly - even if the content is legitimately/legally syndicated and provides user value, it's still possible for Google to consider it "thin", low-value content. There are a lot of caveats, though: (1) It's possible for two or more sites to both rank for a piece of content, but the winner usually depends on a mix of signals, ranging from Google's best guess as to the source to the authority of the domain. In some cases, it's possible for a strong syndicator to outrank the source. (2) It really depends a lot on your content balance. If you use partially-syndicated content in pages with some unique content, you're usually a lot safer. If you use a small percentage of syndicated content across your site, you're also usually pretty safe - worst case, that content may just not rank or get filtered out. The problem is when you rely heavily on syndicated content - it can make your entire site look low-value and trigger issues with Panda. (3) It depends a lot on the type of content. Many resellers use OEM product descriptions, for example. These pages often don't rank well (as they're plastered across the internet), but it's unusual to see them harm the entire site. If you have a blog that's just re-posts from other blogs (even completely legit ones), though, Google is just going to see your site as a copy and low-value. If the content is solely for users and doesn't have SEO value for you, you can always NOINDEX it or rel-canonical to the source (cross-domain). That will prevent an risk. Usually, though, it does have some SEO value, so you don't want to block it entirely. In that case, I would just say proceed with caution. Content syndication is perfectly valid, but you don't want to over-use it.

You can use Moz, Screaming Frog, Sort Site to get results.

I think we were confused about how the actual pages differ? Is the visible content Google is getting different, or is it just source code. Without giving away too many details, can you explain how the content/code is different?

I had it happen in a sitemap I submitted to Google in my Webmaster tools and it didn't like it. So I removed it and resubmitted it.