Rule of Three

Two occurrences may be coincidence; at the third, similarity becomes a pattern — and a moment to consolidate.

The rule of three is a refactoring heuristic: you leave the first duplication alone, and the second as well — only at the third occurrence of the same kind do you consolidate. The reason is twofold: two places don't yet reliably show what is truly shared, so an early abstraction would often be the wrong one; three places make the common shape visible. The rule is therefore not a license to copy but its opposite: it names the moment when copying should stop and consolidating should begin — deliberately rather than by reflex.