1) ALREADY_SUBSCRIBED is returned if attempting to connect to an already connected device that already has an active PCC open with the requesting app. This is an error case and does not return a PCC object; you are expected to continue using the already existing one from elsewhere in the app.
2) I just checked ALREADY_SUBSCRIBED is not currently returned by the request by device number methods because it is missing the proper check. I would expect in the future for this to be fixed.
I will add a note to update the javadoc to make this more clear.
ALREADY_SUBSCRIBED indicates an error in your app not releasing previous instances of PCCs, so you shouldn't have to watch for it if you code your app correctly.