Just as plagiarism is a bad thing, duplicate content can also be. However, it is a nuanced subject. Duplicate content on your own website can be regarded as 'cannibalism' - a situation where any good SEO is cancelled out by competition from another page on your site. So, it's important that you understand the difference between complimentary content and duplicating keywords and concepts.
Duplicating content from another site is plagiarism and is absolutely not ok!
Building authority without duplication
There are ways to add content to your site without duplication. You want to show your authority on a subject by adding content. But it's important to create a broad range of coverage without significant overlap in regards to your keywords.
So, for instance, you wouldn't want two articles on your site with the same keywords or similar versions of these keywords. E.g. "best blenders to buy" and "buying the best blender".
However, you could use "best hard-working blenders to buy" (keywords = 'hard-working blenders'), and "5 things to look for when buying a blender" (keywords = buying a blender).
Use an SEO plugin to help
Using an SEO plugin such as Yoast or All-in-One-SEO is helpful because it alerts you to duplicate keywords being used. This is especially useful if you have a site with a lot of content.
Write complimentary content, rather than competing
The main thing in avoiding duplicate content is to constantly ask yourself - is this content complimentary and not competitive? Are you building a complimentary set of useful information for your customers? If so, you are likely to be fine and not cannibalising any of your own content.
