I'm not sure which answer I should accept and how to proceed.
Then don't accept an answer. It's mentioned a few times in the Help Center:
What should I do when someone answers my question?
As the asker, you have a special privilege: you may accept the answer that you believe is the best solution to your problem.
Accepting an answer is not about whether it is “right” or “wrong”.
As per its tooltip, it should be the answer that helps you the most that you accept, to reward the effort that answerer volunteered to help you.
What you have described is plagiarism. Copying the entirety of someone else's answer is plagiarism, whether cited or not. Flag it as such, with a proper description of what happened, and what the post is plagiarizing.
Of course if someone is using the ideas of another answer (with citation) to augment and improve their own original answer, then that's ...
Of course, otherwise what's the reason of bounties? There are many questions with users, with their answer not accepted, get a bounty, for example (they're only for Python as I searched for Python):
What are metaclasses in Python?
Relative imports in Python 3
How do I lowercase a string in Python?
Of course, anyone can get the bounty.