GREG-17: Duplicate check

This merge request contains a validation check that checks whether two person identities have the same type and value, if the type is not other. This is to avoid duplicates that are not expected in the person identity table.

greg/api/serializers/person.py

@@ -37,3 +37,18 @@ class PersonIdentitySerializer(serializers.ModelSerializer):
     class Meta:
         model = PersonIdentity
         fields = "__all__"
+
+    def is_duplicate(self, identity_type: str, value: str) -> bool:
+        # Guests may be verified using another unrecognised identification method,
+        # which the sponsor is required to elaborate in the value column.
+        # In this case we cannot assume the union of the identity type and
+        # the value to be unique across all records.
+        if identity_type == PersonIdentity.IdentityType.OTHER:
+            return False
+
+        # If the type is a specific ID type, then duplicates are not expected
+        return (
+            PersonIdentity.objects.filter(type__like=identity_type)
+            .filter(value__like=value)
+            .exists()
+        )