Well, I can see the confusion there. If there is already another topic that has been made recently then obviously you dont need to make another one. If it hasnt been posted in in the last few months then thats ok, but only if its a useful post, for example, if someone was having trouble with something and his question was answered by some other members, you dont need to go and bump it with another answer from yourself just worded differently. Also if the topic is useless then it shouldnt be bumped.
If you're unsure PM a mod or admin first, otherwise use common sense.