I have had the same thing happen to me more frequently than ever before. How could a card be offered to me again if it was already accepted in another trade? If I had several trades pending, I always cross-checked them to make sure this wasn't happening. Yet I would end up receiving 2 copies of common cards. I thought about why for a long time and finally, I think I came up with the reason. Some members initiate a trade then leave it in their "Composing" box to continue building it up withmore cards. This can go on for quite some time if the trade # is any indication. When they finally decide to send the proposal, you may have had more trades since they started their offer which also offered you cards that the initial member planned to offer. They would get no indication that you no longer need the card; you would get no indication that you no longer wanted the card.
Lesson: if you get a proposal with a Trade # well below the current trade #, check to see if you need all the cards being offered. An easy way to do this is to open the trade in two windows and Counter / Remove all cards being offered / View all matches / and then by going back and forth between windows, re-add all the cards that was in the initial offer. Any card no longer on your want list will not be in the matches.
Bruno
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Check my Profile page to see my 2023 Goals and my Lists of sets near completion (5 cards or less) or sets getting close (less than 100 cards missing and 75% complete).
https://www.tcdb.com/Forum.cfm/Page/B/ID/0/?MODE=VIEW&ThreadID=25745&C=0