Add duplicate-field checking to darling_core for fields

[TedDriggs]

Right now, the following is valid:

#[derive(FromMeta)]
pub struct Foo {
    #[darling(skip, skip = false)]
    my_field: String;
}

This is probably the result of an error in the crate that's consuming darling, so it'd be nice to tell them. However, it's worth noting that this would be a breaking change, so it needs to wait for a new minor version.