prevent redefining fields in a record

[sophiajt]

Description

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

fixes #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

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 after the PR is merged, if necessary. This will help us keep the docs up to date.