You can see the list of included plugins for X2 at:
http://www.cakewalk.com/Documentation/default.aspx?Doc=SONAR%20X2&Lang=EN&Req=Plug-ins.1.html Here are the included plugins and such for X3:
http://www.cakewalk.com/Documentation/default.aspx?Doc=SONAR%20X3&Lang=EN&Req=Plug-ins.1.html From the above, you will see most are duplicates, but to the best of my awareness you will not cause duplication on your system if you just let the installation rip. It should just overlay ones that were present from prior installations.
There is really no significant gain in install speed by going through the exercise of building the list of ones to exclude from the X3 install, because any processing gains are offset by time taken to build the list and remove checks from boxes.
The above being said, you CAN skip loading Perfect Space, Dimension Pro, and Session Drummer 3, if you already have them installed, and there you will see a reduction in install time, because they have associated content, so since nothing for those changed and if you already have them installed, I recommend skipping them.
Rapture is also at 1.22, and I don't recall when it changed from 1.21, but it is small, so I would just let it install.
Bob Bone