I spoke to an Airbus pilot yesterday, and he confirmed my suspicion that CX676 suffered a dual hydraulic systems fault, based on the flaps and slats not being retracted after landing. A single fault wouldn't prevent normal retraction.

It is possible that the crew decided not to retract for some other reason, but I doubt it.

The risk with a dual failure is that in the event of the need for a go-around, the aircraft cannot be properly configured.

Of course if the third system fails as well, the aircraft cannot fly. However the 3 systems should have no parts in common. There are parts in common for any two of the systems I believe.

Happy to be corrected if someone knows otherwise.