Well, it wouldn't be an adapter "for the Duet," but yes, you'd need a separate adapter to take MIDI into your computerassuming your keyboard does not have a built-in USB interface.
As for what kind of adapter, you'll have lots of choices. M-Audio makes the UNO, which is a one-in/one-out little device for about $40. MOTU makes rack-mount 8-in/8-out boxes that range from $150 on up, just to give you an idea.
I've used my Duet with a wireless MIDI-USB adapter (the M-Audio MidAir, to be exact) and it works fine.
You could always choose an audio interface that includes MIDI, but that would take the Duet out of the running (and I know how pleasant it is to lust after the Duet
)