Hi there,
I just noticed the same thing.
Found a +75 magic damage sword, and when i equip it with my shield, it shows nearly 300 magic damage (instead of 120 without the shield).
Seems like Sigwald said, that the magic damage enchantment is counted twice.
The same thing doesn't occur with normal damage enchantment.
I guess it's a bug, or I'm missing something...
It felt so abused at this moment of the game that i sold it back !!! :-(
