I noticed this yesterday when I helped a user named Christine Hopes on a thread. She first marked my answered correct then marked it incorrect. Then again marked it correct. In this process, I got 25 points twice. Points should be deducted when someone mark an answer correct by mistake and then mark it incorrect. I think same should also apply to Marked Helpful.
Thank you for sharing this behavior with us. I will confer with the technical team for the Community platform and see if this a known issue or expected behavior. I agree that "accidental" markings should be removed from the points total.