Multiple Custom Modules Using Same External JS Lib (e.g. Angular)
The simplest way to solve this is to add a /libraries folder at the root of the Drupal project. Then create a sub-directory for each external library you are using, and reference that from your module.libraires.yml file.

For example, I created /libraries/angular/js, placed the files in there, and then referenced them by starting with a / in the code below.

my-module:
  version: 1.x
  js:
    /libraries/angular/js/angular.min.js: {}
    /libraries/angular/js/angular-sanitize.min.js: {}
    js/my-module.js: {}
  dependencies:
    - core/jquery


Now any module within the project can reference the same files.