I've just looked in the roleplaying board, and your modified post appears as new for me, so this feature doesn't appear to have changed anything. Any post which counts as being edited, and that's all that happens with the double post merging feature, will appear as new to other users.
Double posting remains against the forum rules, but whether it counts as an infringement of the rules when the auto-merge comes into play is currently under discussion. I don't know if I can give an indication of the direction of this discussion, so I'll leave that for someone higher up to comment on.
Double posting is generally not a good idea, but is acceptable in a very small number of circumstances (reaching the character limit, articles and fiction divided into chapters, that sort of thing), so you make be better off sending a PM, depending on how many people are participating in the roleplay. The alternative is to copy your information from your last post if nobody has posted after you, delete your last post, and then copy the information into a new one with the updated information. This is how I would update a roleplay thread if nobody has posted after me.
I hope that helps.