No history of flywheel issues on Up!s, unlike with some other cars (usually diesels as they have high compression ratios).
All Up! engines are triples, and all triples are relatively imbalanced, so have a lot of torque pulsing - a dual mass will stop your gearbox wearing out too soon and give better gearchanges. The Up!/Citigo/Mii all have good, slick gearchanges, especially the post-facelift 5-speed - at least all the ones I've driven!
As far as I know, all common triples have DMFs.
And the Up! is only available as a petrol version I think (at least in the UK).