[ars]

Was this really a dual drive failure, or was this the rather common single drive failure plus undetected errors on a backup drive, that show up when trying to rebuild?

Because that happens a lot, and it's very important to do a full read of every drive in the array at least weekly! You have two options for doing that:

If you are using linux md raid then run the "check" command, which automatically does the test using background I/O (but does still impact things). On debian, and perhaps other distros too the mdadm command will do it every month by default. Make sure to set a minimum speed or it might never finish if you have a busy system.

You can also use the built in SMART on the disk to do a long self test. This also uses background I/O and I think it has a bit less impact on existing operations. (But you have to have some idle time on the disk or it will never finish.) If you install smartmontools you can set smartd to do this test for you every week, and keep an eye on the results.

I personally do both, plus a short self test of the disk every night.