I've started the quest for Onmund to get his amulet back and am at the stage where I need to get the grand staff of charming for Enthir, it's told me where to go but the problem is I've already picked up the staff (when raiding dungeons) before this stage of the quest and so now it won't recognise that I already have it. I'm on the PC so have tried adding the item to the chest and picking it up again but it doesn't recognise it as the Quest Item.

Is there anything I can do to complete this quest?

You can force the quest to the next stage using the command "setstage 6bc33 30", which seems to do the trick. But caveat emptor and all that!

The quest for the Grand Staff of Charming is bugged. But, there is a workaround.

1. Purchase the bedroom set for your house in Whiterun...it includes a Shield and two Greatswords (well worth the 250).
2. Remove one of the swords.
3. Equip the staff.
4. Look at where you removed the sword and a prompt will appear asking you to Activate the rack...press the Activate key/button.
5. The staff gets removed from your equiped hand and gets placed on the rack.

The quest system then picks up the staff and marks it. Then take it to Enthir to complete the quest.

Awful quest in general.

a) he's a jerk
b ) you can't kill him, which you want, because of A
c) if you attack him you get expelled from the thieves guild
d) if you do part C nothing changes, all the dialogue is the same as if nothing happened, and as if you didn't just smack him in the face with an axe
e) you can't pickpocket the amulet off him
f) I found another staff with the exact same name JUST NEXT TO IT. No comment.
g) upon doing this for him since the only other choice you have is not doing it at all, he magically conjures the amulet (where did he keep it? actually, don't answer that.)

The "putting item in weapon rack and then picking it up" fixing the quest is actually super interesting though... It means, if they quickly made quest items droppable, you could probably have a quick work-around for bugged quests.

Sadly, my only hope for peace at mind is mod tools and making the amulet pick-pocketable, and him killable.

LOL, that's hilarious...their attempt at avoiding bugged quests by preventing you from dropping a quest item is actually what is causing the worst quest bugs. I think whoever decided to prevent us from removing quest items from our inventory should be denied the use of the coffee machine until they calm down and think things through at least one step beyond, "I have an idea."

Didnt work for me. I have tried the trick with pacing it on the wall and the "setstage 6bc33 30" and both do nothing.

That's odd. Using the weapon rack should do the trick. Using "setstage" for portions of a quest that involve item possession don't typically work if you already have the item, probably for the same reason item quests don't auto update when you already have said item in inventory. Best thing to do is use a console command drop item, and then pick it up, and that usually does the trick as you've now "acquired" it after the quest stage requiring it has updated. This in particular for those mage book quests you get and already happen to have the book.

Try setstage MGRAppOnmund01 30