Milestones are not repeatable. As stated in the rulebook they are historical moments in your world's history. As they all correspond to something in the Captain's booke it does not make sense that they can be repeated.
We were confused about this as well. In the Captain's book why do some milestones say to destroy them and some don't then?
If they should ALL be destroyed, why bother saying it at all? And then if you are going to say it, why omit saying it other times?
I think this one needs an official ruling.