Redundant information is hard to maintain, keeping it in-sync. Therefore strive for reducing redundancy by defining one authoritative location for each piece of information.
The DRY principle aims is the 'Don't Repeat Yourself' in documentation work. If the same information is added at different locations, it increases the cost of maintenance to track these locations in the case of a change.
Every piece of knowledge must have a single, unambiguous, authoritative representation within a system.
wiki.c2.com. Dont Repeat Yourself
Some form of redundancies may actually help to reduce the documentation costs or support readers in understanding. But these redundancies should be added with care.
In order to reduce redundancy, the information is stored in one location and then reused. The tools to reuse content are transclusion or reference.
References
More information on this principle.
