I just saw this question, so I hope you don’t mind if I jump in here too! 
I know it can be tricky to keep track of responses, especially in very long discussions like the challenges! To the best of my knowledge, the fastest way to check to see if you have already replied to someone is to use the Search Tool (the little magnifying glass 
on the top right of your screen) when you are in a discussion.
If you type in your username and then click “
in this topic”, you will get a list of all your posts and mentions within that discussion- you can quickly see who you already replied to, and anytime anyone mentioned you by name (aka who you might still need to reply to).
To keep things easy to find, you can always reply to multiple people within the same post- it will help you if you ever need to go back and see past conversations! There is a special guide called Replying to Multiple People with instructions about how to do this 
I hope that can help you!
Much love and many blessings 