Trains stops dialog doesn't notice duplicates

```
The train stops dialog allows you to visit the same station twice in sequence 
(A, B, B).  This can be a bug when the user intended to do (A,B,C).  It's 
noticeable in errors when there's no way for cars to get from B to C, but it 
would still be nice to flag obviously erroneous stop lists.
```

Original issue reported on code.google.com by `rwbowdi...@gmail.com` on 27 Jul 2014 at 4:48
