Double. Doctor has two hearts.

[now that he's convinced this Hakuba isn't the person he's looking for, he turns around and heads back to the door, hands stuffed in his pockets casually]

Guess with all the time and dimensional travel we do, it'd make sense that we finally meet our 'doubles.' [it's a phenomenon he was informed of, but had yet to experience]

[he kneels over the lock again with a frown]

Too bad you're not him. I can open this, but it'd be faster with the screwdriver.