I am merely expressing that 'one of the cats are sleeping' and 'one of the cats is sleeping', both should be excepted.
This is a prime example of the psychological and dialectical differences between the U.S. west coast and east coast. West coasters are more pluralisms while east coasters are more singularity oriented (south U.S. more so singularities...). 'are' focuses on the cluster from which the extracted singular came from. 'is' focuses on the extracted singular. Its like in algebra where x = 2y is the same as x/2 = y just the emphases are on x or y relative to the extractable constant, the cat.
I am sure someone could shred me on this in grammar rules but english has several soft dialects (more like degradations lol). Adding this patch to duolingo would be another filling of the gaps for the variance of english speakers.
I would draw the line at: 'one of them cats is sleeps.'
(proceeds to study uninfluenced)