Software Architect's Handbook

DRY – "Don't Repeat Yourself"

The DRY principle stands for Don't Repeat Yourself and strives to reduce duplication in a codebase. Duplication is wasteful and makes a codebase unnecessarily larger and more complex. This makes maintenance more difficult. When code that has been duplicated needs to be changed, modifications are required in multiple locations. If the changes applied everywhere are not consistent, defects may be introduced. Software architects and developers should avoid duplication whenever possible.

When a design violates the DRY principle, it is sometimes referred to as a WET (Write Everything Twice) solution (or Waste Everyone's Time or We Enjoy Typing).