I have noticed when I am posting with my phone it is easy to accidentally tap the screen more than once. NAT has a feature that is supposed to stop making posts quickly. It will come up if posting multiple posts very closely together in time. Tells you to wait so many seconds before posting again. But for some reason if done immediately (like multiple accidental taps on a phone could happen) it will make the posts. 

Might just try being very deliberate when you hit the post button to make sure your finger does not stay near it. That is my best guess of what happens when multiple posts are made accidentally.

Seems like I might have done it once on my home computer when I accidentally clicked the mouse button twice but not sure about that.

As long as it is not the first post in a thread a person can just delete the extra ones. But only the moderators can delete an original first post is my understanding.