Yeah, I ran into this a while back as a DM.... and I let it stack since the defensive appeal of it wasn't too overwealming. I just threw some good flanking archers at them once in a while, and the wizard would get more and more nervous as his images disappeared and the rest of the party was tied up in melee.
In at least one case it added to the 'fear factor' of combat in the outdoors, as an image would just disappear and then the wizard would notice that a rogue had snuck up and tried to backstab, followed by that wizard freaking out and running through a hole in the front lines.