Yes, this is by design. It fires once for each version between the installed version and the one being installed including once for the version you are installing.

This insures that it is always called at least once, even if it has not been installed before.  Otherwise you would not have the oportunity to run code on the installation of a newly installed module.

The expected behavior for a re-install should be that all the steps are ran again just like if it had never been installed in my opinion.  Otherwise the end user would not really be getting a full re-install.  This allows you as the developer to control what needs to happen on a re-install instead of the framework never handing control over to the module.

In other words, if you need to include code to insure that a re-install does not cause errors then it is better that you get the chance to do that then the framework deciding that your code will just never run on a re-install.