My Focus had the over-revving issue. It happened only the once, but that was when the flywheel was dancing all over the place as it was worn out.
A new flywheel cured it. I also had a new clutch fitted too as the old one was >50% worn (84k miles) and was caked in swarf.
I paid £810 (£900 to regular people) for a new Ford flywheel and clutch, fitted by Ford mechanics. This is extremely cheap for main dealer work in Burton On Trent as they have a garage there on the same site as Ford, but with ex-Ford mechanics. The aim is to get other car brands in there, yet they still take Fords?! Makes no sense to me, but they are very good and very well priced when they use genuine parts. Luckily I did not need a new slave cylinder.
A dual mass flywheel can sieze up and act line a single mass one, which will cause torque spikes to go through your gearbox. Not good.
Mine was failing the more common way, with excessive play. Just touch your foot on the clutch, so there is some pressure but it is not depressed. If the engine sounds like a bag of spanners and/or the clutch pedal vibrates violently then the flywheel has excessive play and is dying.
If your injectors start to leak then one day there will be so much diesel in the oil that even when you remove the key from the ignition it will still be revving itself. The fix for this is a £1500+ new injector set. The only thing to do here is to change the oil, so the car runs perfectly again then part-ex it in ASAP.