Plug and Play modems can be installed improperly and produce duplicate entries in the Modem Control Panel (two instances of the same modem). Add sections called PosDup and NoResDup to your PnP modems to prevent this.
If you are writing an INF file for a one-manufacturer, one-model modem device, there is no reason to put a PosDup or NoResDup section in the first draft of your INF file. If you run into duplicate installation problems during testing the installation of your modem, consider adding these sections to your INF file to fix the problem.
Some parallel port driven modems use LPTENUM PNP ID strings. For those devices generally no fix is required along the lines of NoResDup or PosDup.