I found it:
Preferences --> Editing --> Audio --> On Processing Shared Clips
Then, select Open Options Dialog from the dropdown
This should enable the dialog box to show itself when processing audio tracks
Here is a screen capture:
To reverse phase:
Highlight the audio you wish to to have phase reversed then select
Audio --> Process --> Phase Reverse
If the track has recently been copied a dialog box with the following appears:
The project contains events that use the same audio material as the selected! Click “New version” if processing should apply only to the selected events!
Select New version.
Now, the phase reversal should apply ONLY to the copied track (and not simultaneously to the original).