yarn-dedupe

Tools to dedupe yarn.lock

• $ npm install yarn-dedupe -g
• $ yarn-dedupe yarn.lock

Try this:

$ yarn init .
$ yarn add fs-extra@6.0.0
$ yarn add graceful-fs@4.1.6

Now check the graceful-fs package in yarn.lock:

// yarn.lock
graceful-fs@4.1.6:
  version "4.1.6"
  resolved "https://registry.yarnpkg.com/graceful-fs/-/graceful-fs-4.1.6.tgz#514c38772b31bee2e08bedc21a0aeb3abf54c19e"

graceful-fs@^4.1.2, graceful-fs@^4.1.6:
  version "4.1.11"
  resolved "https://registry.yarnpkg.com/graceful-fs/-/graceful-fs-4.1.11.tgz#0e8bdfe4d1ddb8854d64e04ea7c00e2a026e5658"

Why? There're multiple versions of graceful-fs, while the version graceful-fs@4.1.6 can safisfy all sem-versions?If you're creating a React app, when you happily install packages with yarn, you'll get multiple versions of React package sometime, and your app crashes. (Because multiple React instances don't work together.)
And there're so many packages that only work if there's only one version of them.Continue with previous duplicate graceful-fs example:

$ npm install yarn-dedupe -g
$ yarn-dedupe yarn.lock
# read yarn.lock...
# Dedupe result:
 - Resolved: graceful-fs

Now check yarn.lock:

graceful-fs@4.1.6, graceful-fs@^4.1.2, graceful-fs@^4.1.6:
  version "4.1.6"
  resolved "https://registry.yarnpkg.com/graceful-fs/-/graceful-fs-4.1.6.tgz#514c38772b31bee2e08bedc21a0aeb3abf54c19e"

Perfect! We have only one graceful-fs package now!yarn provides locked versions of packages. That's why it won't update existing versions while installing new dependecy. But that may not be what we want.
So this tool is a complementary of yarn lock to manually merge different versions.