I'm not sure about the "members of the same class" part. If, say, a Latin adjective was borrowed twice, once as an adjective and once as a noun (due to substantivisation), then I would still consider it a doublet.

There's also a question of what constitutes a "root word". For instance, aquarium and ewer both ultimately derive from aquarius, but whereas the former is a neutre substantivised form of the adjective, the latter derives from a feminine form. I consider that a near miss.