Duplicate named parameters in route

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

```rust
#[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.

Start in src/lib.rs around the route creation and the duplicate_key test context shown in the issue. Trace how named and wildcard parameters are stored when a route is added, then verify the behavior for repeated names. Done means duplicate parameter names are prevented at route creation and the relevant test confirms the intended result.

Written by the indexing model from the issue text.