mushroom valves redundancy?

[Underwater Tourist]

I am not diving cc, but I read how mushroom valves can be delicate on some units and how they can fail or be accidentally damaged.

Is there a reason why back up mushroom valves aren't used? For example, 1 set of valves at one end of the hose near the mouth piece, and another set of valves at the opposite end of the hose that connects to the scrubber.

Feels like a very cheap redundancy to have?

 

[Tracy]

I am not diving cc, but I read how mushroom valves can be delicate on some units and how they can fail or be accidentally damaged.

Is there a reason why back up mushroom valves aren't used? For example, 1 set of valves at one end of the hose near the mouth piece, and another set of valves at the opposite end of the hose that connects to the scrubber.

Feels like a very cheap redundancy to have?

Everything in the breathing loop creates resistance leading to increased work of breathing. Checking the mushroom valves is part of the build on every unit I am aware of. They don't really fail mid dive. Duckweed on the surface can reek havoc on them, it is something to keep in mind when diving in places with it.
There are two check valves in place to ensure gas exchange with each breath. If one fails, the gas still moves through the loop, you just don't get 100% exchange.

 

[LFMarm]

Checking valves and having spares.