Small update: In order to preserve the errors that got returned by Phoenix’ standard evaluation (size, type, upload count issues), I had to modify the function slightly.

defp mark_duplicate_names(%Phoenix.LiveView.UploadConfig{} = upload, existing_names) do
  processed_entries =
    upload.entries
    |> Enum.map(fn %Phoenix.LiveView.UploadEntry{} =
                      entry ->
      if entry.client_name in existing_names do
        Map.put(entry, :valid?, false)
      else
        entry
      end
    end)

  duplicate_name_errors =
    Enum.map(processed_entries, fn %Phoenix.LiveView.UploadEntry{} = entry ->
      previous_error? = Enum.any?(upload.errors, fn {ref, _msg} -> ref == entry.ref end)

      if entry.valid? or previous_error? do
        nil
      else
        {entry.ref, :name_duplicate}
      end
    end)
    |> Enum.reject(fn val -> is_nil(val) end)

  # Combine existing errors with additional ones evaluated here.
  errors = upload.errors ++ duplicate_name_errors

  upload
  |> Map.put(:entries, processed_entries)
  |> Map.put(:errors, errors)
end