Please note I am basing this on the desktop MPA and 'full version' on mobile - I have yet to experiment with Tapatalk etc.
The main reason content is appearing multiple times is because users are clicking the post new topic/reply button more than once. This is usually as someone may think their content has not gone through - 99% of the time it has.
Therefore, before you hit post thread multiple times (leaving forums untidy/a lot of merging to be done), please go and check in the forum first before you hit the button again. If you are concerned about losing the content in your thread/post then simply copy it so it's in your clipboard. You could even open the forum in a new tab to see if your content is there!
I know this is frustrating, but there is nothing I can do about it - I don't think this is an easy fix for admin either.