The real story is that resource IDs should be unique within a given resource type within a given resource file. When you are adding resources to a resource file, you might want to use UniqueID and/or Unique1ID to help you find unique resource IDs.
However, some resource files do contain multiple resources with the same type and ID. This can be due to bugs in the program which created those files or it can be due to the aforementioned ambiguity in the documentation. When reading resource files your program did not create, make sure your program can gracefully handle such files.
Regardless of what you may read in other documentation, the Resource Manager in some versions of Mac OS may behave unpredictably when reading files with multiple resources with the same type and ID.
One easy way to detect whether a given file contains multiple resources with the same type and ID is to open the file with Resorcerer or ResEdit. These programs will warn you if they detect this condition.