Same Flight Numbers

Tracking a relative’s flight today highlighted something that has confused me a little.

The flight in question is Emirates EK451 from Bali, Indonesia to Dubai. The same flight number (and aircraft) is shown en route from NZ to Bali

Why would two different journeys have the same flight number?

Geffers

Flight originates in NZ and stops at Bali en-route. Not uncommon to retain same flight number for both legs.