Drupal 8 theme showing Uncaught TypeError: $(…).find(…).once is not a function in console New to Drupal 8.

I've created a demo theme. The .libraries.yml file is below:

global-styling-and-scripts:
 version: VERSION
 css:
  base:
   assets/css/font-awesome.css: {}
   assets/css/bootstrap.css: {}
   assets/css/slick.css: {}
   assets/css/bootstrap-datepicker.css: {}
   assets/css/jquery.fancybox.css: {}
   assets/css/theme-color/default-theme.css: {}
   assets/css/style.css: {}
  theme:
   '//fonts.googleapis.com/css?family=Tangerine': { type: external, minified: true }
   '//fonts.googleapis.com/css?family=Open+Sans': { type: external, minified: true }
   '//fonts.googleapis.com/css?family=Prata': { type: external, minified: true }
 js:
  assets/js/jquery.min.js: {}
  assets/js/bootstrap.js: {}
  assets/js/slick.js: {}
  assets/js/waypoints.js: {}
  assets/js/jquery.counterup.js: {}
  assets/js/bootstrap-datepicker.js: {}
  assets/js/jquery.mixitup.js: {}
  assets/js/jquery.fancybox.pack.js: {}
  assets/js/custom.js: {}
  dependencies:
   - core/drupalSettings
   - core/jquery
   - core/jquery.once


If I deleted all the .js from this file, I see no errors in the console however, the images, and slideshows etc are not properly rendering.
You're loading 2 versions of jquery: 
assets/js/jquery.min.js: {} 
- core/jquery 

That's most likely your issue. Try removing the assets/js/jquery.min.js entry and flushing your cache. That will mean you're just using Drupal's core jquery which hopefully your plugin should work fine with