'For a column that is one of multiple pointing to the same table, update
the internal data structures to note that this alias shouldn\'t be used
for those other columns.'

def update_dupe_avoidance(self, opts, col, alias):

    ident = id(opts)
    for name in opts.duplicate_targets[col]:
        try:
            self.dupe_avoidance[(ident, name)].add(alias)
        except KeyError:
            self.dupe_avoidance[(ident, name)] = set([alias])
