Is your feature request related to a problem? Please describe.
Every Stop ID should be unique in stops.txt.

Describe the solution you'd like
Verify a Stop ID is not duplicated in stops.txt

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
GTFS Spec http://gtfs.org/reference/static/#stopstxt