If one kidney suddenly fails, the other kidney usually cannot compensate in time, so labs will show abnormalities. Though even with one healthy kidney, some labs are still abnormal.

But yes, a lot of times, the injuried to the kidneys are happening at the same time (hypotension, drug reactions)

Kidneys are more sensitive to damages and are smaller than other organs as well. On average.

Short of imagining, we do not know which kidney is failing. We can see the overall renal health. If it declines, it will prompt some imagining studies (ie. US, CT).

Why no duplicate? Every organ consumes energy. Imagine if your trunk of your body was twice as big… you need to increase caloric intake and it would make you less mobile. The disadvantage > advantage