This is because I have a lot of .gpx files with many routes and waypoints covering multiple areas.
It is quite possible to accidentally load the same file twice when loading multiple routes via .gpx files for different areas or countries.
The system can already recognise if a waypoint is loaded with the same name (as is normal with any computer system, if a duplicate name is detected then the user is usually offered the option to overwrite or skip).
It would be correct for the route
manager to also recognise duplicate names.
If you are only working with one or two .gpx files then this error may not happen of course.