my_list = [34, 56, 77, 23, 31, 29, 62, 99]
print("The list is :")
print(my_list)
index_list = [3, 1, 4, 6]
my_result = []
for index, element in enumerate(my_list):
   if index in index_list:
      my_result.extend([element, element])
   else :
      my_result.append(element)
print("The result is :")
print(my_result)

The list is :
[34, 56, 77, 23, 31, 29, 62, 99]
The result is :
[34, 56, 56, 77, 23, 23, 31, 31, 29, 62, 62, 99]