Is there any advantage to using one of your planets as a 'trade' planet and building multiple ports around it? I don't have the manual with me as I type this post but I think I remember it merely stating that building a trade port opens up new lanes of trade for increased income (as well as a bonus applied to all connected planets which have a trade port). If you build more than one trade port on a planet does that increase the percentage of trade income the planet contributes to the total?