You can currently define duplicate named parameters in the route with the last one resolved being the end value. E.g. given the following:

#[test]
fn duplicate_key() {
    let mut router = Router::new();

    router.add("/foo/:bar/*bar", "test".to_string());
    let m = router.recognize("/foo/blah/this/is/the/rest").unwrap();
    assert_eq!(*m.handler, "test".to_string());
    assert_eq!(m.params, params("bar", "blah"));
}

this fails with:

thread 'duplicate_key' panicked at 'assertion failed: `(left == right)`
  left: `Params { map: {"bar": "this/is/the/rest"} }`,
 right: `Params { map: {"bar": "blah"} }`', src\lib.rs:339:5

This should probably be prevented when the route is created - given that you can define nameless parameters I can't see a reason for needing to have duplicate names.