If your boyfriend doesn't have a lock on his bedroom
door, I would go to the local hardware store and put
one on. This will keep the other roomate out of his room
with-out permission. In my humble opinion Duncan was
protecting you, when this threating person barged in your
room with out any warning. I am surprise
Duncan did not warn first before a bite occurred.

When you grab Duncan's leash tighter you are sending
a negative signal out to him that something is wrong.
It actually makes him more aggressive.
Now that you know Duncan will bite you MUST
protect Duncan, and others from this happening again.
In my state we have no free dog bites.

If you haven't taken Duncan to training classes,
I would consider going, and also train/socialize him
outside of classes on a daily basis.
That does not mean that strangers need to pet
him, only that lunging and growling in public
is not acceptable just because somebody walks past you.

One of my dogs is animal aggressive, and people weary.
He has never bitten anybody but I also am very careful
not to get in a situation where this might happen.
You always need to think one step ahead of any possible
situation before something might happen. : )