This only works of course if you donl't care about overwritten plist files - which is one of the reasons app bundles don't multilaunch in the first place.
What I personally usually end up doing is command-D, followed by launching the copy. So, there are many solutions to the issue, depending on what you want to do. Its saying that another instance of this installer is already running. Or, you can just wrap the entire bundle inside a platypus application that runs the script you indicate below, for standard double-click multiple execution, from inside the meta-bundle. Platypus is my tool of choice for making launcher wrappers to bypass this - you could try a few different things, such as making an applescript droplet that links a script as you describe when you drop an application on it, or create a folder action script that runs the application with -n when some action happens within that folder. a second launch attempt will bring it to the front. Historically, unlike Windows Explorer, the Finder will only launch an executable binary once. Not really the correct forum for such a question, but something that's doable.
