If you find that two fonts with the same ID have been installed in the same system software, it is most likely the method of installation that is at fault.
System 6 and Before
-------------------
When running System 6.0.x or earlier, always use the Font/DA Mover to copy fonts into the System file. This method will automatically resolve any font ID conflicts that exist. The Font/DA Mover is the only program that should be used to copy a font to or from any file under System 6.0.x or earlier. This ensures that all the necessary resources are copied with the font. The same applies for desk accessories.
System 7
--------
This problem is rare in System 7. The proper way to install fonts in the System file is to drag them directly to the System Folder:
* With System 7.0.x, the fonts will be moved into the System file itself.
* With System 7.1 or later, when you drag fonts to the System Folder icon, they will be moved into a Fonts folder within the System Folder.