Ambiguous or overlapping routes will be overwritten during the build process, so it will never be possible to have more than one route that matches a URL. A message will be printed in the build activity log notifying you when this happens.
The order in which the overwriting of overlapping routes is done is in the sequence in which they are defined in .platform/routes.yaml.
It's best practice to define your routes in such a way to avoid this from happening, and to ensure that no overlaps are possible where route matching is concerned.
For more information on this feature, please see the following page in the Platform.sh documentation: