PAIN
Although Damon knew that the woman presented before him was not Ophelia he did not know how they could get someone that would look exactly like her and also know about them so the time he visited Duane he put him in an illusion to find out about the truth.
Although what Duane thought happened was that he came in and his sweets fell to the floor and Damon started questioning him about how he found Ophelia, but what happened for real was that.
When Damon entered into Duane room he was already there with the sweets on the bed, so after locking him in a trance, he began to ask Duane questions about who the Ophelia that was given to him was, and since the trance he was the questions that Damon asked him was already giving him enough mental pressure for him it was easy for Duane to let out all the secrets.