Ok, avioniker, I start to see the light!
I'm still not 100% sure, but your post suggested me the solution to this dilemma.
Of course we have to thank Boeing for the very bad explanation given to us in the manuals...
What about this: the 9th stage air from the engine is there to help the 5th stage at low rpm, because the 5th stage alone is insufficient at low rpm.
If I increase engine power in a Dual Bleed configuration, the system will think the pressure in the duct is sufficient and will prematurely close the 9th stage modulating valve.
Actually, this is not true, pressure from the 5th stage is still insufficient alone, and APU air pressure will overcome engine air.
This will cause the engine 5th stage no-return check valve to suddenly close, creating a shock in the duct and , as you suggested, possible rupture of the duct itself.
If I increase even further engine power, the 5th stage pressure will be strong enough to reopen 5th stage check valve, overcoming APU pressure: now it's the APU check valve which will suddenly close, creating the same kind of water hammering effect.
Accelerating all this sequence in real time in our mind, we can see how increasing engine power in a Dual Bleed config. will create shocks to both the engine and the APU check valves.
How do you see that?