Handle duplicate column names in CSV import

- display a warning message to the user
- automatically rename columns by adding "__1", "__2", ... suffixes

src/pretix/base/modelimport.py

@@ -47,6 +47,19 @@ class DataImportError(LazyLocaleException):
         super().__init__(msg)
 
 
+def rename_duplicates(values):
+    used = set()
+    had_duplicates = False
+    for i, value in enumerate(values):
+        c = 0
+        while values[i] in used:
+            c += 1
+            values[i] = f'{value}__{c}'
+            had_duplicates = True
+        used.add(values[i])
+    return had_duplicates
+
+
 def parse_csv(file, length=None, mode="strict", charset=None):
     file.seek(0)
     data = file.read(length)
@@ -70,6 +83,7 @@ def parse_csv(file, length=None, mode="strict", charset=None):
         return None
 
     reader = csv.DictReader(io.StringIO(data), dialect=dialect)
+    reader._had_duplicates = rename_duplicates(reader.fieldnames)
     return reader
 
 

src/pretix/control/views/modelimport.py

@@ -146,7 +146,7 @@ class BaseProcessView(AsyncAction, FormView):
         else:
             charset = None
         try:
-            return parse_csv(self.file.file, 1024 * 1024, charset=charset)
+            reader = parse_csv(self.file.file, 1024 * 1024, charset=charset)
         except UnicodeDecodeError:
             messages.warning(
                 self.request,
@@ -155,7 +155,16 @@ class BaseProcessView(AsyncAction, FormView):
                     "Some characters were replaced with a placeholder."
                 )
             )
-            return parse_csv(self.file.file, 1024 * 1024, "replace", charset=charset)
+            reader = parse_csv(self.file.file, 1024 * 1024, "replace", charset=charset)
+        if reader._had_duplicates:
+            messages.warning(
+                self.request,
+                _(
+                    "Multiple columns of the CSV file have the same name and were renamed automatically. We "
+                    "recommend that you rename these in your source file to avoid problems during import."
+                )
+            )
+        return reader
 
     @cached_property
     def parsed_list(self):