I just clicked on SAVE button in the advanced settings for my last post.
Apparently that SAVE wasn't for the ADVANCED settings. It was to save the whole post in drafts.
Then I went to drafts and pulled the post from drafts and posted it.
After it was successfully posted, the app didn't clear the POST status.
When I went to make the next post my previous one was still there in the posting screen with all the fields, each having the same text as before - subject, body and tags.
At that time I didn't know if my post was already submitted and I didn't want to switch back and forth on my phone because I could lose my post.
So I clicked on post and then checked.
Here it go! It duplicated my post.
So now I know that it did post it successfully the first time too and apparently the app simply failed to clear the fields after the successful post.