Software Engineering · Code Quality
The same logic expressed in multiple places in the codebase, so a single change has to be made — and remembered — in every copy.
Example: Fixing a bug in one copy of pasted code and forgetting the other three copies exist.
In practice: Not all repetition is a violation — two blocks that look similar today but encode different business rules can diverge over time, so abstracting them prematurely creates a different coupling problem.