Exception when parsing emails: Key (revision_id, "order")=(2244, 1) already exists.

Another trace back, trying to insert a revision that already exists!

```
Traceback (most recent call last):
File "/srv/patchwork.freedesktop.org/patchwork/bin/parsemail.py", line 758, in main
return parse_mail(mail)
File "/srv/patchwork.freedesktop.org/patchwork/bin/parsemail.py", line 698, in parse_mail
revision.add_patch(patch, content.patch_order)
File "/srv/patchwork.freedesktop.org/patchwork/models.py", line 516, in add_patch
order=order)
File "/srv/patchwork.freedesktop.org/.virtualenvs/django-1.8/lib/python2.7/site- packages/django/db/models/manager.py", line 127, in manager_method
return getattr(self.get_queryset(), name)(*args, **kwargs)
File "/srv/patchwork.freedesktop.org/.virtualenvs/django-1.8/lib/python2.7/site-packages/django/db/models/query.py", line 348, in create
obj.save(force_insert=True, using=self.db)
File "/srv/patchwork.freedesktop.org/.virtualenvs/django-1.8/lib/python2.7/site-packages/django/db/models/base.py", line 734, in save
force_update=force_update, update_fields=update_fields)
File "/srv/patchwork.freedesktop.org/.virtualenvs/django-1.8/lib/python2.7/site-packages/django/db/models/base.py", line 762, in save_base
updated = self._save_table(raw, cls, force_insert, force_update, using, update_fields)
File "/srv/patchwork.freedesktop.org/.virtualenvs/django-1.8/lib/python2.7/site- packages/django/db/models/base.py", line 846, in _save_table
result = self._do_insert(cls._base_manager, using, fields, update_pk, raw)
File "/srv/patchwork.freedesktop.org/.virtualenvs/django-1.8/lib/python2.7/site-packages/django/db/models/base.py", line 885, in _do_insert
using=using, raw=raw)
File "/srv/patchwork.freedesktop.org/.virtualenvs/django-1.8/lib/python2.7/site-packages/django/db/models/manager.py", line 127, in manager_method
return getattr(self.get_queryset(), name)(*args, **kwargs)
File "/srv/patchwork.freedesktop.org/.virtualenvs/django-1.8/lib/python2.7/site-packages/django/db/models/query.py", line 920, in _insert
return query.get_compiler(using=using).execute_sql(return_id)
File "/srv/patchwork.freedesktop.org/.virtualenvs/django-1.8/lib/python2.7/site-packages/django/db/models/sql/compiler.py", line 974, in execute_sql
cursor.execute(sql, params)
File "/srv/patchwork.freedesktop.org/.virtualenvs/django-1.8/lib/python2.7/site-packages/django/db/backends/utils.py", line 64, in execute
return self.cursor.execute(sql, params)
File "/srv/patchwork.freedesktop.org/.virtualenvs/django-1.8/lib/python2.7/site-packages/django/db/utils.py", line 98, in __exit__
six.reraise(dj_exc_type, dj_exc_value, traceback)
File "/srv/patchwork.freedesktop.org/.virtualenvs/django-1.8/lib/python2.7/site-packages/django/db/backends/utils.py", line 64, in execute
return self.cursor.execute(sql, params)
IntegrityError: duplicate key value violates unique constraint "patchwork_seriesrevisionpatch_revision_id_1109258846b6e0ac_uniq"
DETAIL: Key (revision_id, "order")=(2244, 1) already exists.
```