Had a question come up in my latest play of this. Can you trade people cards that they can not technically take. ie. I use my trader to give you purple dancer, even though you already have a purple dancer.
Based on what I have read this is a legal move, just take cards that can't be inserted into your guild hall and place them in the discard, but it is not all that clear in the rules. If you could help with this, that would be great.
You can do a swap which will move cards to your guildhall which duplicate cards in your play area (played this turn) which have not yet been moved to your guildhall, and in this case the duplicates are silently discarded when they are to be added from your play area at the end of your turn.
This is the only exception to the no-duplicates rule, though. You can't ever cause duplicates in your or another person's guildhall.