`--execute-params` does not check uniqueness of keys

I have:

• searched the issue tracker for similar issues
• installed the latest version of Quarto CLI
• formatted my issue following the Bug Reports guide

Bug description

It appears to be possible to have duplicated keys in the YAML file given to --execute-params. The last key wins.
As for any other YAML, I think Quarto should error out.

Steps to reproduce

A Jupyter example but same is happening using knitr as the engine.

---
engine: jupyter
---

```{python}
#| tags: [parameters]

trans = "log1p"
```

```{python}
trans
```

With YAML file:

trans: "log10"
trans: "log2"

quarto render index.qmd --execute-params test.yml

Actual behavior

Quarto takes the YAML and only the last duplicated keys is used without any warnings/errors.

Expected behavior

An error or at least a warning.

Your environment

No response

Quarto check output

Quarto 99.9.9
[✓] Checking environment information...
      Quarto cache location: /Users/mcanouil/Library/Caches/quarto
[✓] Checking versions of quarto binary dependencies...
      Pandoc version 3.6.3: OK
      Dart Sass version 1.87.0: OK
      Deno version 2.3.1: OK
      Typst version 0.13.0: OK
[✓] Checking versions of quarto dependencies......OK
[✓] Checking Quarto installation......OK
      Version: 99.9.9
      commit: b5438495f9605043dc96e006d494efee5e850dea
      Path: /Users/mcanouil/Projects/quarto-dev/quarto-cli/package/dist/bin

[✓] Checking tools....................OK
      TinyTeX: v2025.05
      Chromium: (not installed)

[✓] Checking LaTeX....................OK
      Using: TinyTex
      Path: /Users/mcanouil/Library/TinyTeX/bin/universal-darwin
      Version: 2025

[✓] Checking Chrome Headless....................OK
      Using: Chrome found on system
      Path: /Applications/Brave Browser.app/Contents/MacOS/Brave Browser
      Source: QUARTO_CHROMIUM

[✓] Checking basic markdown render....OK

[✓] Checking Python 3 installation....OK
      Version: 3.13.1
      Path: /Users/mcanouil/Projects/quarto-dev/quarto-playground/.venv/bin/python
      Jupyter: 5.8.1
      Kernels: julia-1.11, python3

[✓] Checking Jupyter engine render....OK

[✓] Checking R installation...........OK
      Version: 4.5.0
      Path: /Library/Frameworks/R.framework/Resources
      LibPaths:
        - /Users/mcanouil/Projects/quarto-dev/quarto-playground/renv/library/macos/R-4.5/aarch64-apple-darwin20
        - /Users/mcanouil/Library/Caches/org.R-project.R/R/renv/sandbox/macos/R-4.5/aarch64-apple-darwin20/4cd76b74
      knitr: 1.50
      rmarkdown: 2.29

[✓] Checking Knitr engine render......OK

Start by reproducing the issue with quarto render index.qmd --execute-params test.yml, using the shown duplicate-key YAML and checking both Jupyter and knitr engines. Trace the --execute-params handling and add regression coverage; done means duplicate keys produce an error or warning instead of silently using the last value.