It seems to me that linking to an already existing thread (if there is one,and if I know about it), is a good practice, particularly if there is already a discussion ongoing. Not only does it avoid taking away from the ongoing discussion, it's also a good way to give credit to members who find new things, which of course encourages finding new things. Given the numerical difference between the membership and the mod team, it's the members that find most new stuff these days, and that sort of thing is both helpful and to be encouraged.
As for closing the new, front page thread, I tend to not do that, for a couple of reasons. The first is that some folks simply want to respond on the front page thread. This is particularly so if the post is ended in such a way to provoke a respose, or the responder may want to address something particular in the post itself, which can get complicated to do in another thread. Another reason not to close is that it looks kinda funny if front page posts have no responses. Random folks just wandering by might get the (very much mistaken) idea that there's not much traffic here from such a condition.
All that being said, there's not really a hard-and-fast rule or even rule-of-thumb to refer to on this, because the situations will necessarily vary, both from topic to topic and person to person. Looking back over the front page items that have pre-existing threads in the time I've been around, it seems to me that more of the front page threads are "open" than not, mainly for the sorts of reasons I mentioned.
