Do you know which is the biggest misconception about duplicate content?
Most of the people think that by publishing content taken from other websites you can incur in duplicate content penalties and loose google rank.
Actually, it doesn’t work like that.
What Google defines as duplicate content penalty is tied to content appearing on a same website on more than one page (be careful, this is valuable also on different subdomains on the same domains).
Anyway, you cannot take anybody’s content and republish it under your own name, this would be against copyright laws.
Let’s see how you should behave following the rules.
Articles
If you take an article from an article directory and put it on your website, there will be no duplicate content penalty, if you don’t publish it on more than one page. On the contrary, if the article provides good quality content, Google will love it.
If dealing with articles, be sure to add the resource box along with it, or you will incur in some more serious copyright problems.
This way, the author of the article will love you too, since you’re providing him one more free one-way link.
PLR content
The same concept works for PLR content, except for the resource box step. In this case you can claim full authorship on the content.