This is a great question because a lot of people think that every website has to be 100% original.
The truth is the typical website has, at least, some level of duplication.
In many cases, it can't be avoided.
The key here is to ask yourself: “What is the ‘tolerable’ threshold for my site?”
And, unfortunately, that doesn’t have a straight answer that is good for all sites and for all time.
You have to look at the context of your site.
What kind of website do you have? What kind of content do you produce?
More importantly, what kind of niche are you in?
If you are, for example, in a news niche, and you basically syndicate other people’s content, and you add a little bit of commentary here and there, that is to be expected.
And guess what?
Google will still rank you!
If you don’t believe me, just look at all the syndicated content that Google shows in search results all day, every day.
But, in other niches, duplicate content is the kiss of death.
So pay close attention to what other players are doing in your niche, and take a look at the specific context in which duplicate content can make sense in your situation.
Ideally, you should just feature original content.
But, depending on how your site is structured, this is either inconvenient or simply impractical.