Unable to trade 2:1 because "You have already created that offer before"
I found a bug that was pretty irritating. What happened was; I was unable to trade 2 Trees for a Stone. I had the Tree Port - and I was able to trade for other resources at the 2:1 rate - but it would only attempt to trade to the other players when I attempted to do so. Eventually I'd get the "You have already created that offer before" dialog.
The only connection I can make is that I also had the Stone Port? Shouldn't matter obviously; but its the only possible area I could guess at causing the programming issue.