Well, thats' why you NEVER connect that second lux to your trade network BEFORE trading out the original. Only then you connect the surplus. Problem solved.
A lux is 5 extra pop points to the AI (playing on Cheiftain happiness settings) - empire wide.
Luxes no longer have the same weight they used to have in Civ3 with its huge multiplier possibilites with a market or Civ4, both of which affect each city. Most of the time, they become repeat customers and you can charge them for the next 30 turns. You can also wage economic war by witholding a repeat trade later on once they become reliant on your luxury for happiness, especially those 'friends' who ramp up their conquests later on and are heavily dependent on any happiness resource they can find.
I find once I allowed myself to mentally be ok with giving away free luxuries to friends, my diplomatic game has improved greatly, because you're actually signalling to the game in concrete ways that you are a friend to them. None of that roleplay stuff where you pretend to be their friend and make up a story why that was the case.
Being scrooge mcduck over 240 gold or maybe even only 7 gpt for 30 turns is not worth it. And oftentimes, the trading opportunity is not present anyways.