Duncan's Shield is a 'gift' item that can be equipped once given to Alistair. Giving this gift triggers a short cut scene.
History
Spoiler Alert
In order to get the shield, the PC must speak Ser Riordin, the Orlesian Grey Warden. He is located in the Arl of Denarim's dungeon & can be found during the quest to save Queen Anora. After his rescue, he can be found in Arl Eamon's study, where the PC has the ability to ask him about the Grey Warden Cache. The cache is located in the Marketplace Warehouse, in Denarim.
Trivia
The PC has the option to speak with Alistair about the subject of Grey Wardens. During this conversation, if the PC asks about Duncan, Alistair will reveal that he wished he had something of Duncan's to remember him by.