Well, the only situation I could think of is if you've got 2 trading partners, 1 of them has a luxury resource that you want, but doesn't need and of the resources that you've got multiple copies of. You could trade one of your additional copies to the other trading partner to get another copy of "your" luxury and then trade your original copy to the other AI. You'd end up with +4 happiness, some diplomacy bonuses from trading but you'd also lock another of your luxuries in the process.
Aside from that really rare scenario I can't really thing of any scenario where it would be useful.