Admin form crashes when limit_choices_to query returns non-distinct results

[Karolis Ryselis]

consider the following models:

class Warehouse(models.Model):
    branch = models.ManyToManyField(to="Branch")


class Branch(models.Model):
    title = models.CharField(max_length=32)


class Order(models.Model):
    warehouses = models.ForeignKey(to="Warehouse", limit_choices_to={"branch__title__startswith": "A"}, on_delete=models.PROTECT)

All models are registered using default admin.ModelAdmin.
Consider the following data entered:
Branch with name "A branch"
Branch with name "Another branch"
Warehouse related to both branches.

When trying to add a new order, the same warehouse is duplicated two times in the select box. When trying to save it, it crashes with error

get() returned more than one Warehouse -- it returned 2!

The function that crashes is apply_limit_choices_to_to_formfield. It performs a complex query with whatever we passed to limit_choices_to in field declaration and the result is not a distinct queryset. It is possible to work around this by creating a form for Order, overriding its __init__ and adding self.fields["warehouse"].queryset = self.fields["warehouse"].queryset.distinct(). However, I think that this should at least not crash.

[Tim Graham]

Duplicate of #1891.