duplicate chunk labels emit error

[LiNk-NY]

Hi!

Thanks for the great resource the cookbook is.

This section in the cookbook states that one can use the same chunk label in another chunk.
https://bookdown.org/yihui/rmarkdown-cookbook/reuse-chunks.html#same-label

When I try doing this in practice, I get an error.
Here is an example with a file test.Rmd:

---
title: "Testing Rmd Format"
vignette: |
  %\VignetteIndexEntry{Testing Rmd Format}
  %\VignetteEngine{knitr::rmarkdown}
  %\VignetteEncoding{UTF-8}
---

```{r loading, eval=FALSE}
# this is a chunk
1 + 1
```

```{r loading, eval=TRUE}

```

Leads to the error:

> rmarkdown::render("test.Rmd")
processing file: test.Rmd
Error in parse_block(g[-1], g[1], params.src, markdown_mode) : 
  Duplicate chunk label 'loading', which has been used for the chunk:
# this is a chunk
1 + 1

The error also occurs when running R CMD build on a package.

PS. I am aware of options(knitr.duplicate.label = 'allow') but this is not possible for external build systems (e.g., CRAN).

Thanks!