Duplicate content in the SEO space has been an important topic for as long as I've been in the industry. I've covered it here countless times.
Today, I wanted to share how Google's John Mueller says what "effects you'd see with content duplication within a website are."
John says there are two main issues with duplicate content:
(1) Google's algorithms will choose one URL to show for the content in search. Maybe it won't choose the URL you'd choose. If you have a preference, make it known (through redirects, rel=canonical, internal links, etc).
(2) Depending on the amount of duplication (is each piece of content hosted 2x, 20x or 200x?), it can happen that the process of crawling is too much for the server, or that new/updated content isn't picked up as quickly as it otherwise might be.
John said in most cases, in cases of "reasonable amount of duplication" and with a "reasonably strong server", these are not issues. John said in these cases, "neither of these are real problems. Most users won't notice the choice of URL, and crawling can still be sufficient."
Forum discussion at Stack Exchange.