Don't repeat yourself

Dave Thomas says it nicely on this web page.

"DRY says that every piece of system knowledge should have one authoritative, unambiguous representation. Every piece of knowledge in the development of something should have a single representation. A system's knowledge is far broader than just its code. It refers to database schemas, test plans, the build system, even documentation."

Refer To

• Once and only once
• Duplicate code smell
• Code smells
• Refactoring