Improve error reporting for duplicate CSV column headings

Fixes #11990

netbox/utilities/forms/utils.py

@@ -198,8 +198,12 @@ def parse_csv(reader):
         header = header.strip()
         if '.' in header:
             field, to_field = header.split('.', 1)
+            if field in headers:
+                raise forms.ValidationError(f'Duplicate or conflicting column header for "{field}"')
             headers[field] = to_field
         else:
+            if header in headers:
+                raise forms.ValidationError(f'Duplicate or conflicting column header for "{header}"')
             headers[header] = None
 
     # Parse CSV rows into a list of dictionaries mapped from the column headers.

netbox/utilities/tests/test_forms.py

@@ -319,6 +319,22 @@ class CSVDataFieldTest(TestCase):
         with self.assertRaises(forms.ValidationError):
             self.field.clean(input)
 
+    def test_duplicate_header(self):
+        input = """
+        status,status
+        Active,Active
+        """
+        with self.assertRaisesRegex(forms.ValidationError, 'Duplicate'):
+            self.field.clean(input)
+
+    def test_duplicate_header_key(self):
+        input = """
+        vrf.name,vrf.rd
+        Test VRF,123:456
+        """
+        with self.assertRaisesRegex(forms.ValidationError, 'Duplicate'):
+            self.field.clean(input)
+
     def test_clean_default_to_field(self):
         input = """
         address,status,vrf.name