deserialize w/ a tag + catch-all (flatten) surprisingly duplicates tag

[michaelkirk]

You can use flatten to capture additional fields.
You can use tag on a struct to add a field during serialization. This tag is also used for discriminating enums during deserialization.

But when you use them together, it seems as though the tag gets redundantly processed into the flattened field.

My actual use case is georust/geojson#269, but here's a simpler version:

use serde::{Deserialize, Serialize};
use serde_json::json;

use std::collections::HashMap;

#[derive(Debug, Serialize, Deserialize)]
#[serde(tag = "type")]
struct Foo {
    name: String,
    #[serde(flatten)]
    other: HashMap<String, String>
}

fn main() {
    // case 1
    let input = json!({
        "type": "Foo",
        "name": "Frank",
        "bar": "bar_value"
    }).to_string();
    let foo: Foo = serde_json::from_str(&input).unwrap();
    // Prints `type` as member of `other`:
    //       GOT> Foo { name: "Frank", other: {"type": "Foo", "bar": "bar_value"} }
    // But I'm expecting no explicit type field.
    // EXPECTING > Foo { name: "Frank", other: {bar": "bar_value"} }
    println!("deserialized: {foo:?}");
    
    // case 2
    let wrong_input = json!({
        "type": "WrongType",
        "name": "Frank",
        "bar": "bar_value"
    }).to_string();
    let foo_wrong: Foo = serde_json::from_str(&wrong_input).unwrap();
    // Successfully parsed! Prints `type` as member of `other`:
    //       GOT> Foo { name: "Frank", other: {"type": "WrongType", "bar": "bar_value"} }
    // But I'm expecting `serde_json::from_str` to fail, since `type` doesn't match the expected tag value ("Foo").
    println!("deserialized with wrong type: {foo_wrong:?}");

    // case 3
    let serialized = serde_json::to_string(&foo).unwrap();
    // Prints TWO type fields:
    //       GOT> {"type":"Foo","name":"Frank","type":"Foo","bar":"bar_value"}
    // But I'm expecting only ONE type field:
    // EXPECTING> {"type":"Foo","name":"Frank","bar":"bar_value"}
    println!("serialized: {serialized}");
}

This seems similar to #1786, but my example is a little different because it includes a flattened member.