Collections: Sets, bags or something else?

A follow-up to my previous:

The spec appears to carefully use "collection" and not "set".  This,
together with the absence of expression equivalence rules like /a+a=a/
and /a*a=a/ and the note that assertions of the same /type/ may occur in
an alternative, suggest that "collection" here means "unordered
collection with duplicates allowed", informally known as a "bag".

Is this the intended meaning?  It's not unheard of to use "collection"
to mean "set" (i.e., duplicates are not considered).  If the intended
meaning is to allow duplicates, is there any special meaning to the same
/alternative/ appearing more than once in a policy (as opposed to the
same /assertion/ (type?) appearing more than once in an alternative,
which behavior is out of scope).