Sub prefixes duplication.

Greetings!

Any way to use prefix which contains different sub prefix without duplication?
What I mean:

use config::{Config, Environment};

fn main() {
    std::env::set_var("FOO_BAR_FIRST", "1");
    std::env::set_var("FOO_SECOND", "2");

    let conf = Config::builder()
        .add_source(Environment::with_prefix("FOO_BAR").separator("_"))
        .add_source(Environment::with_prefix("FOO").separator("_"))
        .build()
        .unwrap();

    println!("{}", conf.cache);
}

Got: { first => 1, bar => { first => 1, }, second => 2, }.
Expected: { first => 1, second => 2, }.

Start by reproducing the Rust example with Environment::with_prefix and the two overlapping prefixes, then trace how environment keys are merged into Config. Done means the resulting cache contains first and second without the duplicated nested bar entry, while preserving both environment values.