I thought MPGH had one but guess not. I see double post happening multiple times to many different users. It mostly happens when if you have slow internet connection and you might click reply more than once and it'll double post.
This mod will just automatically merge your double posts. You can also do thread bumping which is useful in Marketplace sections. It will automatically merge your posts and still bump the thread so it'll stop people those who just bump to gain more post counts. You can ignore/disable auto merging on some of the forums also.