The Paste function at first goes to the original track whence the copied/cut clip had come. To change that, you need to mess with the Auto-Select buttons -- the black dots to the right of the track names. OPTION-(number pad 0-9) would toggle the audio ones. Hit them so that the desired target track is the first one with the dot on it -- for example, A1 trumps A2, A2 trumps A3, etc.
Two even faster ways to do what you're describing:
1. With the default Arrow tool, hold down OPTION before you click on the clip you want to duplicate. With OPTION still held down, pull it down a track and now also hold SHIFT to do an Overwrite instead of an insert. That duplicates the clip without moving the original.
2. Double-click the clip into the Viewer. Now drag it from the Viewer back onto a track beneath. Since it's audio, you'll have to drag it while holding onto the "Hand" icon (right side of a Viewer with an audio clip in it).
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