Registering single shift-watch when rule exists already throws internal error

error seems here:
```python

File "/app/tapir/shifts/views/views.py", line 396, in form_valid

    return super().form_valid(form)
```
throws e.g.
```

django.db.utils.IntegrityError: duplicate key value violates unique constraint "shift_watch_constraint"
DETAIL:  Key (user_id, shift_id)=(634, 12023) already exists.

```