InstallAware

I have an installer that installs a few small applications. Some of the applications use the same dll. In my installer, each application is a component/feature.

If I make each component install all the files necessary for it to run, will those shared files be duplicated inside the installer's executable? Will the install executable try to install the file twice (once for each component)?

Example:
Component foo and component bar are selected to install. Both install to the same directory. Both use the file jimmy.dll. Does the installer have two copies of jimmy.dll? Does the installer write jimmy.dll for component foo and then have to overwrite jimmy.dll for component bar?

Depends on how the dual inclusion occurs, but in most cases, yes - there will be two duplicate copies. If you wish to avoid this, you can add the file as a support file/creative, and then use the Copy Local Files command for that file from $SUPPORTDIR$ instead of using Install Files. Or, you can just install the file once using Install Files based the conditions for it to be installed being present or not.

Lets say that I have three programs that use a shared file. If I use the Copy Local Files command to copy that file to where I want it, would the installer try to copy it 3 times, or would it just try to copy it once because it isn't executed until Apply Install?

One more question: We use compiler variables to make the build script play better with our version control software. At first glance, it appears that the support files can not have compiler variables in their path. Is that true?

It would just make one copy.
Yes, support files do not support compiler variables.