Well you weren't the only one..I see 7 items in the database with duplicate lisings in the current event from 2 people besides you. I see the listing times are right on top of one another too so I'm thinking maybe you (and the others) where a little quick on the submit button and hit it it more than once on the same screen when it didn't come back fast enough.
The duplicate entries were harmless since the trades are recorded by the item's id, not by the listing's id so I just deleted the duplicates. If it happens again let me know. You should also be OK if you remove it (which would remove both) and add it again.
If you find it doing it to you consistantly and you're not clicking the save/submit button twice let me know. I didn't want to fool around with it in a live event so I just fixed it and moved on.
Thanks.
--Kevin