There isn't an ADP5428 listed on the MFI techical disc, there is an ADP5426 which is cream 850mm dual fuel range cooker.
I've used the ADP5426 wiring diagram.
To have both oven heat up when switched off would require a fault on the controls for both ovens.
On the wiring diagram it shows two thermostats, one to control the top and one to control the bottom.
It would require both of these to be faulty to cause the problem you describe.
The cure would be to replace the thermostats.
The only other possible cause would be a wiring fault.
Have you had work carried out on the appliance just prior to noticing this fault?
When it goes wrong are the temperature lights lit?
Hope this is of some help.
Let me know how it goes or if you need more help.