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-protocol/src/shell_error.rs

@@ -597,6 +597,20 @@ pub enum ShellError {
         src_span: Span,
     },
 
+    /// Fields can only be defined once
+    ///
+    /// ## Resolution
+    ///
+    /// Check the record to ensure you aren't reusing the same field name
+    #[error("Record field used twice")]
+    #[diagnostic(code(nu::shell::not_a_list))]
+    ColumnDefinedTwice {
+        #[label = "field redefined here"]
+        second_use: Span,
+        #[label = "field first defined here"]
+        first_use: Span,
+    },
+
     /// An error happened while performing an external command.
     ///
     /// ## Resolution