It says on the wiki that sometimes The Jagged Crown isn't a quest item and you can duplicate it with the mannequin glitch.

http://elderscrolls.wikia.com/wiki/The_Jagged_Crown_(Item)

"Sometimes the Jagged Crown isn't marked as a Quest Item, allowing you to use the manniquin reproduction bug to accure a second Jagged Crown, however if you have it on your person when you finish the quest, both Jagged Crowns will be removed from your inventory."

Can anybody tell me how to do this?

Edit Ps: my crown is locked.

Guess not...