The option is set to upgrade older versions.
Our installers are in a version control (SVN) system, during the install we set the version number at build time, and this "changes" the aip file, and marks it as modified within the version control system, so after the build we revert the changes (as part of our automated scripts), so its likely that this causes the product code to change for every build (even on the same version)
To me it seems a gap in advanced installer that you dont detect the same version number with a different product code, and you should really be covering these holes.
if you have a greater version number you get a message
if you have a lesser version number you get a message
if you have the same version number (but different product code due to using AIC commands during a "rebuild") then, no message and it installs a duplicate copy.