There are multiple modules with names that only differ in casing.

TL;DR When using npm link don’t change the casing of your paths.

Another misleading error message that hopefully will help someone out quicker than I managed it.

WARNING in C:/Users/SebastianRogers/Repos/Correspondence Management App/node_modules/process/browser.js
There are multiple modules with names that only differ in casing.
This can lead to unexpected behavior when compiling on a filesystem with other case-semantic.
Use equal casing. Compare these module identifiers:
* C:\Users\SebastianRogers\Repos\Correspondence Management App\node_modules\process\browser.js
Used by 1 module(s), i. e.
C:\Users\SebastianRogers\Repos\Correspondence Management App\node_modules\path-browserify\index.js
* C:\Users\SebastianRogers\repos\Correspondence Management App\node_modules\process\browser.js
Used by 5 module(s), i. e.
C:\Users\SebastianRogers\repos\Correspondence Management App\node_modules\babel-loader\lib\index.js!C:\Users\SebastianRogers\repos\Correspondence Management App\src\repository\sharepoint-data-repository.ts
@ C:/Users/SebastianRogers/Repos/Correspondence Management App/node_modules/process/browser.js
@ C:/Users/SebastianRogers/Repos/Correspondence Management App/node_modules/path-browserify/index.js
@ ./node_modules/mime-types/index.js
@ ./src/service/msgraph-service.ts
@ ./src/index.ts

The issue here is not the modules as stated its actually the path taken to reach the modules.

The root of the application on my development machine is:

C:\Users\SebastianRogers\repos\Correspondence Management App

However I had swapped development machines and the previous machine used:

C:\Users\SebastianRogers\Repos\Correspondence Management App

Mostly it didn’t make a difference but for this one module it did.

Why?

I had downloaded the Microsoft Authentication Library for JavaScript (MSAL.js) locally, built it and linked it into the project so I could understand what was going on.

The solution was to unlink it and then relink it on the new machine.

TL;CR When using npm link don’t change the casing of your paths.