don't allow duplicate contents. Why? imagine a reader will read again what they have read on other site they have visited before.
Surely they will come to this scenario "I HAVE READ THIS SOMEWHERE ELSE" now confusion starts to build up pin pointing which one is legit and original.
And if I am the one writing that content or the author and I found out someone copied my writings for the use of their own personal benefit I would feel bad, it's plagiarism.
Search engine impact
It will affect you hard...duplicate content are a big no even if you will rephrase it. Search engines are getting smarter now and can justify low quality content by their algorithm.
A good example is when it autocorrects your searches like DID YOU MEAN "your query" so it exactly knows correct grammar and spelling.