Over the last 20 or so years, I've worked on many forums as both a Moderator and an Administrator, using several different forum software packages, some of which are free like PHPBB and some are very expensive, high quality programs like vBulletin and Invision Power Board. The duplicate posts thing happens on
all of them, and it has nothing to do with how you post. (Computer, phone, etc.) For this reason, I think it's a PHP bug. Whenever I see a duplicate post or thread, I just delete it. In the case of duplicate threads, if both of them have posts in them, I usually try to merge them. But that sometimes leads to threads that don't make a whole lot of sense if you don't know that the threads have been merged. So I usually add a comment at the end of the first post letting people know that the threads were merged so that they won't think that everyone that posted in it was, you know, like stoned or something...
