psycopg2.errors.UniqueViolation: duplicate key value violates unique constraint "e2e_one_time_keys_json_uniqueness"

This issue has been migrated from [#7365](https://github.com/matrix-org/synapse/issues/7365).

---

Storage function `add_e2e_one_time_keys` claims that it raises an error when one tries to insert a key that already exists in the db. This makes a call to `/keys/upload` fail if one of the keys in the set is already known:

```
synapse.http.server - 110 - ERROR - POST-xxx - Failed handle request via 'KeyUploadServlet':
...
File "/synapse/storage/data_stores/main/end_to_end_keys.py", line 237, in _add_e2e_one_time_keys
for algorithm, key_id, json_bytes in new_keys
File "/synapse/storage/database.py", line 684, in simple_insert_many_txn
txn.executemany(sql, vals)
File "/synapse/storage/database.py", line 202, in executemany
self._do_execute(self.txn.executemany, sql, *args)
File "/synapse/storage/database.py", line 225, in _do_execute
return func(sql, *args)
psycopg2.errors.UniqueViolation: duplicate key value violates unique constraint "e2e_one_time_keys_json_uniqueness"
DETAIL: Key (user_id, device_id, algorithm, key_id)=(@blabla:example.com, ATNXJDJWK, signed_curve25519, AAAANR) already exists.
```

Perhaps this in intentional, but presumably we'd want to just skip a key if we already have it and continue inserting the rest rather than 500 (well, ideally we wouldn't do that in any case).