DRY = Don’t Repeat Yourself

the only way to develop SW that is reliable, easy to understand and maintain
every piece of knowledge must have a single, unambiguous, authoritative representation within a system

The alternative

the same thing is expressed in two or more places
if you change one, you have to remember to change the others
it isn’t a question of whether you’ll remember: it’s a question of when you’ll forget

DRY concerns

code
documentation - don’t describe what the code does in comments
data
developers - encourage active and frequent communication (stand ups, Slack channels, code reviews)

Source