Adding microchip numbers to patients helps to avoid duplicate patients. If you try to add a new patient with a microchip number that has already been saved to another patient's records, a warning is shown on the patient details editing page.
Note, however, that it is possible to save a duplicate microchip number without a warning to an existing patient from the microchip icon, on the 'mark as arrived' page, or when adding a microchip number with the microchipping procedure. You can avoid this by adding a microchip number from the patient details page, where a warning is always shown if the microchip number is already in use.