If you've ever had to pry a spent round out of a chamber because the single extractor jump the rim,you would appreciate dual extractors my friend,
why wouldn't one work if the other one is broken? Although I'm sure their suppose work together and are not intended to act as a back up by any means.
And again,Mossbergs pass a Milspec teat of 3000 rounds of FULL-POWER buckshot with out any breakage or failures. And were the only one to do it.
Was that the test for the Marines and a new breaching SG for urban ops?
Those tests are notoriously skewed. Look at how the Navy DEVGRU and SFG Alpha/Delta guys all wanted the new HK416 (IIRC) and it was the only one that passed a test, but some loudmouths and some congressman raised a fuss made them use US built or something and are still "issued" the M4's etc. Of course, they are creative and still have the 416's stashed I hear.
"Although I'm sure their suppose work together and are not intended to act as a back up by any means."
I think that is the point. It relies on 2 for ejection. If one is bent, burred, or damaged you get no love.
Think tweezers with one side bent? Personally I haven't experienced it happening, but have it on good authority that it can be an issue.
As far as quality goes I shot a qual at a neighbouring agency with a new 870 out of the box with ghost rings, alternating slugs and shot all day. It was flawless, literally out of the UPS truck I was told, And supposedly inspected before sent out for duty! I shot 4th and the f'ing guys in front of me on the line didn't tell me the sights were way off! So my first 2 at 50 were off, but I figured it out from the snickering. Everyone else got a pass as the sights got done by an armorer during a break.

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But it went the whole day like a slave without a FTF/FTE. 2000 rounds. 100 reps of 20 rounds each per 870, 4 on the line. 8000 examples that day of how the 870 just works.