Jest node module resolution bug when using browser field

How to reproduce

git clone git@github.com:lhorie/jest-browser-bug.git
cd jest-browser-bug
yarn
yarn test

Expected

Test should pass, console should log 1

Actual

Test fails, console logs 2

Explanation

Running node index logs 1.

The index.js file requires two modules: ab and ac, both of which require a third module aa. The aa module is symlinked from a monorepo-like structure and not hoisted to the top level node_modules. The folder structure looks like this:

- aa
  - index.js
- node_modules
  - ab
    - index.js
    - node_modules
      - symlink to aa
  - ac
    - index.js
    - node_modules
      - symlink to aa

When jest runs w/ the option browser: false, node module resolution works as intended: it detect that ab/node_modules/aa and ac/node_modules/aa are both symlinks to the same physical folder and run the aa module only once.

However, when browser: true, ab/node_modules/aa and ac/node_modules/aa are considered to be different packages and run aa twice.

This is problematic for several reasons:

top level side effects occur more times than they are intended to, potentially leading to race conditions if they touch network or filesystem
exported entities lose reference equality (thus failing checks like ===, instanceof, etc that they would otherwise pass)
symbols no longer work across dependent modules