Document how to resolve "Duplicate graphql modules" issue as part of onboarding

[JofArnold]

It's in the console but would help to set expectations.

[benjie]

See also: graphql/graphql-js#594

Detecting:

$ find node_modules -name graphql
node_modules/graphql

The above should only return one result. If there are other versions (in submodules) then they need to be "removed" so that only one version exists.

Resolving

Resolving with yarn: https://yarnpkg.com/lang/en/docs/selective-version-resolutions/

Resolving with npm: ???

Resolving with webpack:

  resolve: {
    alias: {
      graphql:  path.resolve('./node_modules/graphql'),
      react:    path.resolve('./node_modules/react')                // Same issue.
    }
  }

From the horse's mouth

Quote from Lee Byron, creator of GraphQL:

graphql-js is not designed to support using functionality between different installations of itself. The instanceof is only one example where this failure occurs - but there are others as well, especially if there would ever be inadvertent differences in released versions.

If you use packages with sub-packages, ensure they support ranges of versions for graphql so that npm or yarn can find a single version to install at the top level which works for all dependents.