First let's define duplicate content. Having the same content on different pages of the same site. This is the key.
But, you can have the same content on different sites. The best way is to see this in action.
Google a popular review site. Then copy a paragraph from the review.
Paste the paragraph in the search bar with quotation marks at the beginning and end of the paragraph. This means Google will search for sites with these exact words only.
Google will search a maximum of the first 32 words in the search bar.
Then you will see the different sites that contains these exact words. This was an eye opener for me, I always thought Google didn't like duplicate content, until I understood what duplicate content actually was.