There is a difference between random.choices() and random.sample() in Python

python_random_sample.md

In python, random.choices() will select K items from a list with replacement, which means that there are may be duplicated items in the result list. If you want to select K random item from the list without duplication, you need to use random.sample() instead.

I wasted one hour debugging this issue and suddenly found the casue. Sad :[