In version -09, the specification has been restricted so that the multiple
associations continue in parallel (i.e. 'merging' is not allowed). The
circumstances under which you can get multiple associations are in fact rare:
there need to be multiple routes *in use* between the same pair of nodes, which
implies some form of policy routing or load sharing.

There is an issue if nodes attempt to economise on the number
of messaging associations by retaining a single one where several
have been set up with the same authenticated identity. at the
moment the spec says you MAY do this but doesn't indicate how
each end agrees this should be done (if one end doesn't agree,
some validation checks will fail and messages will be rejected.)

I lean towards a MUST or MUST NOT here, with updates to the 
text on validation in the former case.