What version of Audition are you using?
I'm pretty sure that keyframes duplicate when you duplicate a track in Audition CC
I certainly agree that directly entering a value for a keyframe would be MOST useful - here's hopeing.....
Duplicating a track does duplicate the envelopes and keyframes in my version of Audition CS6 in the Multitrack view. However if you want to replace the audio clip place and adjust your keyframes in the Track Envelopes lane as opposed to the on Clip envelopes which get deleted with the clip if you replace it.
Very 'sound' advice