It is expected behavior. Your code is equivalent to:

_result = []
for i in your_list:
    _result.append(your_list.append(i))

which is equivalent to:

_result = []
_j = 0
while _j < len(your_list):
    i = your_list[_j]
    _result.append(your_list.append(i))
    _j += 1

It is an infinite loop (because len(your_list) is increased after your_list.append(i)), and two lists grow with every iteration.