You get 4 happiness from the first instance of a luxury you have, and no additional happiness from copies. Copies of luxuries are only useful (1) for their tile yields, if you have a citizen working those tiles, (2) as spare resources in case your initial copy get pillaged or is otherwise lost (e.g., lose city in a war) or (3) for trading with the AI (which, by the way, does not impair your ability to get tile yields (see (1)). So, in general, unless you are rabidly concerned about (2), you should be trading your excess luxuries for whatever the AI will pay -- whether that's gold, some strategic resources, a copy of a luxury you don't have, etc.
Moderator Action: Since this is not a bug, thread moved to General Discussions.