input: Write a python function to find duplicate values in columns of an Excel sheet.
  assert find_duplicates("Customers.xlsx", "Customers", 0) == ['John Doe', 'Jane Smith']
  assert find_duplicates("Customers.xlsx", "Customers", 1) == ['123 Main St', '456 Elm St']
output: def find_duplicates(file_name, sheet_name, column_index):
      import openpyxl
      wb = openpyxl.load_workbook(file_name)
      sheet = wb[sheet_name]
      values = [sheet.cell(row=i, column=column_index + 1).value for i in range(1, sheet.max_row + 1)]
      duplicates = [value for value in set(values) if values.count(value) > 1]
      return duplicates