Duplicated values when flattening an enum.

[morr0ne]

When flattening an enum the values aren't discared and they get deserialized twice.
Here is a reproducible example:

use serde::Deserialize;
use serde_json::Value;

#[derive(Debug, Deserialize)]
struct Parsed {
    name: String,
    #[serde(flatten)]
    status: Status,
    #[serde(flatten)]
    unknown: Value,
}

#[derive(Debug, Deserialize)]
#[serde(untagged)]
pub enum Status {
    Dead { hour: u32 },
    Alive { age: u32, residence: String },
}

fn main() {
    let json = r#"{
        "name": "Ben",
        "age": 10,
        "residence": "Planet Earth"
    }"#;

    let parsed: Parsed = serde_json::from_str(json).unwrap();

    println!("{parsed:#?}")
}

link to playground

This outputs the following:

Parsed {
    name: "Ben",
    status: Alive {
        age: 10,
        residence: "Planet Earth",
    },
    unknown: Object({
        "age": Number(
            10,
        ),
        "residence": String(
            "Planet Earth",
        ),
    }),
}

The enum values get parsed both in the status field and the unknown field.
The same thing doesn't happen when using a struct:

use serde::Deserialize;
use serde_json::Value;

#[derive(Debug, Deserialize)]
struct Parsed {
    name: String,
    #[serde(flatten)]
    status: Status,
    #[serde(flatten)]
    unknown: Value,
}

#[derive(Debug, Deserialize)]
pub struct Status {
    age: u32,
    residence: String,
}

fn main() {
    let json = r#"{
        "name": "Ben",
        "age": 10,
        "residence": "Planet Earth"
    }"#;

    let parsed: Parsed = serde_json::from_str(json).unwrap();

    println!("{parsed:#?}")
}

link to playground
Which outputs the following:

Parsed {
    name: "Ben",
    status: Status {
        age: 10,
        residence: "Planet Earth",
    },
    unknown: Object({}),
}

I'd expect the unknown field to be an empty object in both cases.
This is clearly a bug as extra data is being parsed which clearly should not have been.

This bug is clearly caused by the way the derive attributes generate deserializing logic.
It also happens on other formats (I actually encoutered this problem while implementing my own format), I chose serde_json to make the examples more readable.

[morr0ne]

I think this is the same issue highlighted in #2200, I didn't notice at first because of the weird wording in the title.

[kangalio]

Yes, this seems to be a duplicate of #2200

[Mingun]

Even more: this is duplicate of #1909