new form model in apps.dashboard.forms to handle existing item-lists; new test model in .tests.test_forms to accommodate; also new method in .test_views, but skipped for now

src/apps/dashboard/forms.py

@@ -1,6 +1,8 @@
 from django import forms
+from django.core.exceptions import ValidationError
 from .models import Item
 
+DUPLICATE_ITEM_ERROR = "You've already logged this to your list"
 EMPTY_ITEM_ERROR = "You can't have an empty list item"
 
 class ItemForm(forms.models.ModelForm):
@@ -20,3 +22,14 @@ class ItemForm(forms.models.ModelForm):
     def save(self, for_list):
         self.instance.list = for_list
         return super().save()
+
+class ExistingListItemForm(ItemForm):
+    def __init__(self, for_list, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        self.instance.list = for_list
+
+    def clean_text(self):
+        text = self.cleaned_data["text"]
+        if self.instance.list.item_set.filter(text=text).exists():
+            raise forms.ValidationError(DUPLICATE_ITEM_ERROR)
+        return text