prevent redefining fields in a record (#8705)

# Description

Prevents redefining fields in a record, for example `{a: 1, a: 2}` would
now error.

fixes https://github.com/nushell/nushell/issues/8699

# User-Facing Changes

Is technically a breaking change. If you relied on this behaviour to
give you the last value, your code will now error.

# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass
- `cargo run -- crates/nu-utils/standard_library/tests.nu` to run the
tests for the standard library

> **Note**
> from `nushell` you can also use the `toolkit` as follows
> ```bash
> use toolkit.nu # or use an `env_change` hook to activate it
automatically
> toolkit check pr
> ```

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

crates/nu-command/tests/commands/reject.rs

@@ -85,7 +85,7 @@ fn reject_record_from_raw_eval() {
     let actual = nu!(
         cwd: ".", pipeline(
             r#"
-            {"a": 3, "a": 4} | reject a | describe
+            {"a": 3} | reject a | describe
             "#
         )
     );
@@ -98,7 +98,7 @@ fn reject_table_from_raw_eval() {
     let actual = nu!(
         cwd: ".", pipeline(
             r#"
-            [{"a": 3, "a": 4}] | reject a
+            [{"a": 3}] | reject a
             "#
         )
     );