def add(self, dist):
        """Add `dist` if we ``can_add()`` it and it has not already been added
        """
        if self.can_add(dist) and dist.has_version():
            dists = self._distmap.setdefault(dist.key, [])
            if dist not in dists:
                dists.append(dist)
                dists.sort(key=operator.attrgetter('hashcmp'), reverse=True)
Add `dist` if we ``can_add()`` it and it has not already been added