Re: Option to Renumber Posts in a thread (fix for double-post)
Actually, the issue with double (triple etc) posts (the ones GMs users can't just delete) is that all of the posts have the same message number. They're supposed to be unique, and the system can't handle it as it cannot uniquely identify the post you're after.
Similarly, being able to renumber posts won't solve this issue either, as you'll run afoul of the same duplicate post ID issue.
Renumbering posts will, as Skald mentioned, break links, but it will also mess up the chronological order of the posts which (if you move the last post in a thread to not be the last) will break thread read/unread tracking. Certain assumptions are made when threads are displayed, such as the last message in the thread is the last message that was posted to the thread.
Post links aren't an insurmountable problem (you just have to be aware that it will break them) and background calculations can (probably) overcome the break in chronological order, but it's not as easy as it sounds. Lack of testers we have also means that new versions are very slow to test and roll out (without increasing the chance of bugs).