Creating two sections in different chapters with the same name appears to link all table of contents links back to the first section with that name.

Steps to reproduce:

bookdown-bug-demo.zip

1. Download the demo here: https://bookdown.org/yihui/bookdown/get-started.html
2. In 01-intro.Rmd add to the end:

## Visualisation

Test

3. In 02-literature.Rmd add the same thing to the end (so the section labels are the same: "Visualization".
4. Build using the "Build" button.
5. Open the resulting HTML file and click on the table of contents link for the "Visualization" section of Chapter 3. Notice that it takes you to Chapter 2 instead.

What I expected: The link to the "Visualization" section in Chapter 3 should take you to Chapter 3.

Some things to note:

• Naming the "Visualization" section in 02-literature.Rmd to "Visualization2" fixes the issue.
• I have confirmed this also occurs with the development version 0.20.1.
• Reverting to bookdown 0.19 fixes the issue.

[I note offhand that I found this issue when installing the development version to try to fix another issue last week. It slipped my mind reporting it because it turned out my other issue was with another package...]

> xfun::session_info('rmarkdown')
R version 4.0.0 (2020-04-24)
Platform: x86_64-apple-darwin17.0 (64-bit)
Running under: macOS Catalina 10.15.5, RStudio 1.3.1023

Locale: en_GB.UTF-8 / en_GB.UTF-8 / en_GB.UTF-8 / C / en_GB.UTF-8 / en_GB.UTF-8

Package version:
  base64enc_0.1.3 digest_0.6.25   evaluate_0.14  
  glue_1.4.1      graphics_4.0.0  grDevices_4.0.0
  highr_0.8       htmltools_0.5.0 jsonlite_1.7.0 
  knitr_1.29      magrittr_1.5    markdown_1.1   
  methods_4.0.0   mime_0.9        rlang_0.4.6    
  rmarkdown_2.3   stats_4.0.0     stringi_1.4.6  
  stringr_1.4.0   tinytex_0.24    tools_4.0.0    
  utils_4.0.0     xfun_0.15       yaml_2.2.1     

Pandoc version: 2.9.2.1

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