IntegrityError: (raised as a result of Query-invoked autoflush; consider using a session.no_autoflush block if th...

https://sentry.io/organizations/hypothesis/issues/856736679/?project=37293&referrer=github_plugin

```
IntegrityError: (raised as a result of Query-invoked autoflush; consider using a session.no_autoflush block if this flush is occurring prematurely) (psycopg2.IntegrityError) duplicate key value violates unique constraint "uq__user_group__user_id"
DETAIL: Key (user_id, group_id)=(225407, 40059) already exists.
[SQL: 'INSERT INTO user_group (user_id, group_id) VALUES (%(user_id)s, %(group_id)s) RETURNING user_group.id'] [parameters: {'group_id': 40059, 'user_id': 225407}]
(35 additional frame(s) were not displayed)
...
File "sqlalchemy/util/compat.py", line 202, in raise_from_cause
reraise(type(exception), exception, tb=exc_tb, cause=cause)
File "sqlalchemy/engine/base.py", line 1182, in _execute_context
context)
File "sqlalchemy/engine/default.py", line 469, in do_execute
cursor.execute(statement, parameters)
File "newrelic/hooks/database_psycopg2.py", line 51, in execute
**kwargs)
File "newrelic/hooks/database_dbapi2.py", line 25, in execute
*args, **kwargs)
```

Start with the linked Sentry event and the reported SQLAlchemy stack trace, then trace the application path that inserts into the user_group table. The issue is resolved when the duplicate user/group relationship no longer produces this IntegrityError, with a regression test or reproducible verification.

Written by the indexing model from the issue text.