Speed

When making a full attack action, the wielder of a speed weapon may make one extra attack with it. The attack uses the wielder’s full base attack bonus, plus any modifiers appropriate to the situation. (This benefit is not cumulative with similar effects, such as a haste spell.)

Moderate transmutation; CL 7th; Craft Magic Arms and Armor, haste; Price +3 bonus.

The problem is it is not a stacking issue. The enchantment is affecting the weapon not you. Each enchantment on each weapon is affecting a different thing and so normal stacking principles do not really apply. Now I think it should and the entry should say that the target is affect by the extra attack portion of the haste spell so there would be no way of stacking.