Forbid duplicate meta items in `#[state_machine]` attribute

[mdeloof]

At the moment the #[state_machine] attribute will accept the following:

#[state_machine(
    state(derive(Debug)),
    state(derive(Clone))
)] 
impl Foo { ... }

Only the last state meta item with the Clone derive will be used and the state meta item with the Debug derive will be ignored.

To keep the macro code simple I suggest we forbid the use of duplicate meta items.