Duplicate content is an issue that has terrified many people, but which is vastly misunderstood.
It was introduced by search engines to filter out duplicated web page content on the internet.
The purpose of a search engine is to provide a useful service to its customers, those that use it to seek information on the internet.
The customer does not want to be presented with a list of results that are all the same, each web page a duplicate of the other or near enough so.
If you were searching for information using a search engine, you would want each result provided to be different from the other.
Let's take Google as the example, since it is normally the search engine that people talk about when discussing duplicate content.
Google looks for similar content between different web pages, either across domains or within a single domain.
When it finds it, the algorithm will decide which of the duplicates is the most relevant, and hence most useful to its customer, and filter out the rest.
A website is not 'punished' for duplicate content, simply filtered out so that only the best is left.
Google quotes the example of regular and printable versions of the same content.
If you have that on your website, then unless you block one of the pages from the spiders, Google will decide itself which to list.
It will not list both.
However, if it perceives the duplication as being deliberately designed to 'manipulate rankings', then the website might have its ranking and listing position 'revised'