Every piece of knowledge in a system should have a single authoritative representation.
Hunt and Thomas's 1999 formulation. The cost of violating it is not the duplicate itself but that the copies drift, and nobody can then say which is authoritative.
In practice
- A VAT rate is written into four services, changes in a budget, and two of them are still charging the old figure a year later.
- A company's leave policy appears in the handbook, the intranet and the offer letter, and an employment tribunal gets to decide which one is authoritative.