Can multiple panels with the same code be used?

Yes, if they share the exact same design (code lengths, width, height and other settings). However if panels share the same code then the app won't be able to tell the difference between them. This means that finding out the machine (or wall, etc) on which the panel is applied is not possible anymore. In general it is considered good practice to keep panel codes different even if the same UI is to be overlaid on them.