Duplicate paper titles

This code identifies nearly 600 duplicate paper titles:

```py
titles = set(); duplicates = set(); idmap = {}
for p in anthology.papers():
t = str(p.title)
if t in titles:
duplicates.add(t)
else:
titles.add(t)
idmap.setdefault(t, set()).add(p.full_id)
for t in sorted(' '.join(sorted(idmap[t])) + ' ' + t for t in duplicates):
print(t)
```

#8659 addresses 186 of these. That appears to be the only duplicated proceedings.