If you have 'new info' to add to the thread that you feel should be posted separately to your first post or perhaps you want to 'bump' the thread to get more attention to it (after hours have passed for example) then, go ahead and double post HOWEVER, double posts will be merged together by mods/admins when they feel they should be. I personally do that once the obvious reason for the double post has passed (eg if you are updating the thread and a few people have posted since then so theres no need for the double post) or I'll post in the thread myself to keep it active then merge the double post.
Hell, Im confusing things arent I?
Basically, on forums generally it is seen as a bad thing to double post. On GirlGamers, its your choice to double post or not (you do what you think is best for the thread) and our staff will manage the double posts that arent needed.