How to quickly duplicate a track.

1. Start by disabling your video track so you don't duplicate the video.

2. Navigate your playhead to the clip you'd like to copy duplicate

3. Press Cmd/Ctrl + C to Copy

4. Then move the existing track or tracks to a new track.

5. Make sure you have cleared your original tracks completely. 

In this case the audio clip was on audio tracks 1 & 2. It is now on audio tracks 3 & 4, leaving audio tracks 1 & 2 empty.

6. Set your playhead back to the beginning of your selection.

7. Press Cmd/Ctrl + V to paste

8. Now your track is duplicated. Remember to lock it to your existing track if you want your duplicate to move with the original.

9. You can also lasso select multiple clips and Press Cmd/Ctrl + C to copy them.

10. Just remember to put your playhead back at the beginning of your first selected clip before you paste in your duplicate.

11. Press Cmd/Ctrl + V to paste

12. This process can be used with any number of clips.