Extend _clean_fields to catch MultipleObjectsReturned and display a toast with a useful message instead of a 500 error

netbox/netbox/forms/base.py

@@ -2,6 +2,7 @@ import json
 
 from django import forms
 from django.contrib.contenttypes.models import ContentType
+from django.core.exceptions import MultipleObjectsReturned
 from django.db.models import Q
 from django.utils.translation import gettext_lazy as _
 
@@ -99,6 +100,12 @@ class NetBoxModelImportForm(CSVModelForm, NetBoxModelForm):
     def _get_form_field(self, customfield):
         return customfield.to_form_field(for_csv_import=True)
 
+    def _clean_fields(self):
+        try:
+            return super()._clean_fields()
+        except MultipleObjectsReturned as e:
+            self.add_error(None, f'A non-unique value was provided for one or more fields: {e}')
+
 
 class NetBoxModelBulkEditForm(CustomFieldsMixin, forms.Form):
     """