In OpenACS, an object (e.g. a package) can be mounted multiple times in the site map. Although this might not be the "usual" case, it is certainly considered a supported feature, as one can do this not only via the API but even through the site map user interface by first creating a folder and subsequently mounting an existing package instance.
With that said, I propose to deprecate the proc site_node::get_node_id_from_object_id
, which suggests to the developer that node-to-object is a 1:1 relationship, in favour of a new proc site_node::get_node_ids_from_object_id
.