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1/3 of my article is copied from another site. Will that constitute to duplicate content?
You have to change that part of article or content. You have to write unique content again for that part of website. Google highly dislike any duplicate website for making penalty.
One third of your content is copied from another site? Oh My God! your content is not just a duplicate content, but also a plagiarized content. You might be penalized by Google. In order to avoid penalty, you need to at least rewrite the portion you copied.
I guess even if you copied only 1 long sentence that will constitute a duplicate. I have been reading about duplicates and the advice when you cite a line from another website is to enclose it in quotation marks to mean you have copied it legitimately because you cited it in your article. But without the quotations that would be tantamount to copied content from another site.