My understanding is that if he has access to all luxuries you have, he will like you more. If you have a single luxury that he does not have access to, he will like you less. This would imply that if you trade one of the copies of a resource to him and keep a second for yourself, this should qualify as a positive modifier. Since the map usually distributes luxuries with large lumps of the same type in localized areas, my strategy has been to trade the first copy that I improve to Monty and quickly improve a second for myself.
There are several problems with this. First, if you become suzerein of a city-state and get luxuries from them, he will dislike you. This is why it's really annoying that you can't trade copies obtained from city-states to others- if they happen to have multiple copies you can't benefit in any way, and if those copies are the only ones you have access to, Monty will dislike you. From experience, I'm pretty sure this also applies to unique luxuries (Zanzibar.) Second, you get a pop-up warning message from him the instant you improve the resource, not at the end of the turn like all other warnings. This would seem to imply that the modifier is activated before you could possibly trade it to him- you can't trade it before it is improved and he doesn't give you an opportunity to do so before the pop-up.Third, unless you have a user interface that notifies you when deals have expired, you need to remember the turn that your deals with him expire so you can renew the deals. Even then, I'm pretty certain that it's like the second problem I mentioned- when the turn begins, the negative modifier will be activated before you have a chance to trade with him. Fourth, while you'd often try to increase your amenity base by trading for luxuries with other civs, you will receive a negative modifier with Monty if he isn't also trading for that luxury, and you can't prompt him to trade for it, even if the third party has extra copies.
All that said, I think
@Lord Lakely has a point about how negligible the modifier is. I've had games where I've relentlessly ensured that he has access to every single luxury that I had, and he hated me anyway. I've also had games where II ignored this aspect of his traits altogether, and we were two peas in a pod.
