Disabling mail would basically break a design loop in the game as the AH would no longer work fundamentally (sales or expired items get sent to you via mail also anything purchased is sent via mail so you cannot GET anything you bought). lso you would need to be doing excessive traveling or expensive (for newer players) ports to clan mates to do crafts for them or give hand-me-downs so that they can use it (as you can't sell the item on the AH anymore as teh AH would not function as previously stated). It would also make resource acquisition almost impossible for certain things as well as gear (suzette, the solution, fuzzy bunnies, crimson mask etc).
Whilst there are indeed issues with server stability, removing/disabling mail would make the game such a painful experience.
How to fix any duping things? I have no idea, it all happened after I left the game, but I'm sure they are working on a solution thats not going to fundamentally change and potentially break the games progression curve for leveling and trade skill grinding (and end game gear too when we get there)(