A Monero transaction input's ring signature contains a set of one time output keys corresponding to the outputs of some previous transactions. Such a transaction can have multiple inputs, and therefore multiple sets of old outputs. It would not make sense for any one old output to appear more than once in the bunch of inputs, since an output can only be spent once.
Within a single input's ring, does the core Monero implementation allow duplicate ring members? Within all the rings combined, can a member appear more than once? If either of these rules hold, are they held by the protocol or merely by the standard implementation?
This question corresponds to protocol v12. Were the answers above different in previous versions? (this is not necessary to answer, just curious)