I'd be fine with shields having a normal chance for creation. I would even be fine with them all applying. However, the highest DR should be the only one that applies to damage. As in, a Thorn Carapace and a Frost Shield would both have a chance to freeze and poison upon attack, but they only have a DR of 2 from Frost, not a combined DR of 3. Also, if the attack misses as in via Dusk Shield, no effects should apply, for example procrastination.
Alternatively just have it replace the slot if it becomes something of that type.