Thanks for your response.
In fact, I decided that it is not possible to have the formatting that I described, and I simply went through my document and added suffixes as appropriate, and removed the 2nd reference(s). However, this, it seems to me, is not how EndNote should need to be manipulated.
The publisher I am sending my article to is American Geophysical Union (published by Wiley).
Maybe EndNote could be updated in the future to include the type of formatting I want?