I haven't got the game installed, but I think there's a setting in GlobalDefines you can change.
The way dagriggstar puts it, with <update></update> and <where></where> is how you would do it in a proper mod, to make clear to the game what needs to be changed, but for a proper mod you'd need ModBuddy as well, it's bit more work.
It's easier to go directly into your game files and change that value. Normally that's not recommended, changing the original files of the game, but if it's just one number, that's not too bad.
But Civilization never gave extra happiness from more instances of the same luxury type, there are good game reasons for that. I wouldn't easily change that.