Don't believe him, he don't know SEO, but he is a "good" marketer, market newbies, okay no hate here. If anyone really want to see a big fault of his blog, I posted one of his biggest fault in 2016 maybe I forgot in BHW here. Again, no hate, he don't know seo, nowadays he just hire, or write some plain things, which everyone knows.
And for duplicate content, there are some circumstances:
1) Google indeed punish duplicate contents, want to test out? Buy a domain, post 1,0000 duplicate posts, and you will see what you get in some months or even weeks, pure spam message in your GSC.
2) If you just have "some" extend duplicate posts here and there, its normal, maybe u won't get a hard hammer, just your rankings maybe "not that good" as original ones
3) syndicate posts to social medias is normal, if post is not indexed but u quickly share them on social medias and got indexed there, it may pose some problems, sry I remember read sth about this topic on seroundtable and searchenginejournal but forgot the conclusion, ( either there is a "soft" duplicate not a big problem, or your social media post will rank better than your original domain post, again, I forgot it, anyone can check them, if I am right, john muller answered this)
PS: I read another interesting thing regard this topic from a good source: for example, an authority website copied your original news or post, and ranked better than u, people found that it seems the original post will get "a bit of authority" in such a case gradually.
again, not my found, judge it by yourself.