So try this:
Ele bota, comandando, a sua rosa,
até encontrar a sua segunda bota.
Uma rosa é uma rosa é uma rosa ...
And you are right. In this case, without further context, both singular and plural answers should be accepted.
Moreover, the common scenario is for the commander to have a PAIR of boots, not just one. The case where one is talking about only ONE boot is a special case (needs context).
But this is typical Duolingo, not thinking through what a sentence can mean. Very inconsistent.