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In tumblr, it's easy to reblog posts that you want posted on your page. Are they considered duplicate content by Google?
Yeah. Google will count that duplicate content. When you are doing re blog on tumblr than you are copying another website content to your tumblr blog. Google show the search result by two factors- content and back link. If content goes for duplicate than that tumblr blog will not come easily to Google search engine result area.
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If Tumblr reblogs were considered duplicate content NOBODY WOULD REBLOG ON TUMBLR! Three things you need to be a successful Tumblr blogger: followers, likes and reblogs. The reblogs always point back to the original source link. That's why reblogs are not considered duplicate content by Google.