Redundancy

Database normalization. Software refactoring. Approaches to reducing redundancy in various types of systems.

Biology. Language. Systems that depend on certain sorts of redundancy to function correctly. Understanding what someone is saying over a noisy telephone line.

Systems where redundancy is good vs. systems where redundancy is bad.

What are the good kinds of redundancy?

In systems where redundancy is good: finding ways to improve the quality of the redundancy, rather than just adding more.