Okay. After dueling extensively I've isolated four distinct symtoms.
1. Falling off the End of the World
2. The "slide" where you are pushed away from the opponent
3. Teleporting where your opponent (or you) disappears for a second and reappears in a different spot sometimes with the target spot in the old location.
4. Magic just doesnt work ... this is unique to dueling and as I am told you are well aware of it.
Again, the multiple key strokes of this kind of combat (slash and dash) seems to be the culprit

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Good luck with this one
Post Script; [Please please please write some response to this thread so I know you have seen it...combat is a healthy potion of everyones gaming experience, having this resolved - to me - should be a priority

]