If this is intended, then the draft might express the constraint more clearly.
It also would appear to work counter to the modularity goal, that distinct
processes be able to augment a document without knowledge of the prefixes used
by other processes. It's just the flip-side of different URI's bound to the
same prefix: if the URI's already have registered prefixes in order that the
application can pick the uniquely correct one, then the registry can constrain
the prefixes to avoid collisions.
If you intended to say, that two distinct prefixes may not be simultaneously
associated with a given URI, that's a slightly different story, but it has the
same ending.