Blade of cunning is the lvl 10 rogue quest. You should only be able to complete the rogue quest one time. If you got it on SW and again on darnassus then did them both. I would suspect it is working as intended. You should not be able to loot that weapon two times. I also believe if you abandon the quest after completing on in a different city that you wouldn’t be able to get it again.
If you destroyed it and you no longer have it, i would ask a gm to try to restore it. That weapon is going to last about 7 or 8 lvls for you.