Per the documentation you could try passing new="window" to open a new window every time (e.g. use functools.partial or lambda to bake in the different param value and pass the wrapper to add_callback).
However, please be advised that Server.show is really just a very thin wrapper on top of the Python stdlib webbrowser.open function. That function has a lot of platform-dependent, 'if possible", caveats. There is no guarantee that that what you ask for is possible on all platforms, or on the platform you happen to be using. YMMV and you will just have to try and see.