my_list = ["python", "is", "fun", "python", "is", "fun", "python", "fun"]

print("The list is :")
print(my_list)

my_result = [value + str(my_list[:index].count(value) + 1) if my_list.count(value) > 1 else value for index, value in enumerate(my_list)]

print("The result is :")
print(my_result)

The list is :
['python', 'is', 'fun', 'python', 'is', 'fun', 'python', 'fun']
The result is :
['python1', 'is1', 'fun1', 'python2', 'is2', 'fun2', 'python3', 'fun3']