This gets even more interesting with coding agents.

A second copy is no longer just maintenance debt. It can become a second source of truth that an agent confidently follows after the original has changed.

I've seen the same rule copied into CLAUDE.md, AGENTS.md, tool-specific config and project docs. Keeping those synchronized is already hard for humans; agents just make the stale copy actionable.

So I increasingly think DRY applies to knowledge and instructions at least as much as it does to code: one canonical source, references everywhere else.