You can't delete a post. Once someone replies to a question it gets taken out of the unanswered queue. That's the best you can do. You can do that by replying to your own question. Other online forums might work differently.
In your case, you could have avoided the duplicate post by going back and adding to your original question, instead of posting a new question. Click the three dots in the top right corner of your question, then click Edit Message.
