Reguarding those repeating commodities, I could speculate that each supplied/demanded item has a numeric value that once reached, supply/demand is (satisfied), and a second numeric value that when reached supply/demand changes, and that those values depend on things like size of the city and number of trade arrows, but that is about the best I can do with my emperical evidence.
Reguarding demand in a city, I've found that at least some of the time, it is based on the size (or maybe size of the trade potential) of the cities involved. In one game I had an AI capitol demanding dye. With some experimentation, I found that if i delivered the smaller payload dye caravans first, I could deliver more dye caravans before the AI demand changed or was satisfied. There seemed to be a certain size to which the dye changed to something else demanded, anything over changed the demand, anything under left it still in demand in the sink city.
Conversely, other than Hides, I've never had a repeat commodity to deliver without at least 3 trade routes already in the supply city. When I had 3 routes predelivery, and the city i was delivering to had less trade than the smallest trade city, it often would simply free up the commodity to trade again (i 'got rid' of the caravan, without filling in a trade route). The rest of the time, one of the supply commodities changed, sometimes to what I had just delivered.
Also, I've had small trade cities deliver lucrative caravans, gold to another continent for example, (satisfy) demand in that city, where its other commodites have no noticeable effect, so the factors that determine trade bonus and whether the trade route replaces an existing one are likely part of the equation when dealing with repeating commodities.
Hope I didn't just confuse everyone more than they already were
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April Cantor: Sire, in order to expand further we must first gain favor of the King
SCG: darn, I've never really got the hang of that tribute thing, guess it will be a long time until i make prince
*goes off and starts gifting gold and techs*