subscribe the entire pd-list to pd-announce (so all announcements go to pd-list and thus to all pd-list subscribees) AND disable the direct delivery of pd-announce for all those addresses that are also subscribed to pd-list.
some users might have set up a filter to throw announce messages in a seperate folder, via procmail or gmail's 'labels' based on from/to:pd-announce; sending announces to the user list in addition, (but sending an additional one if the user subscribes to announce explicitly) would be my vote.