How to fix "Uncaught TypeError: $(…).owlCarousel is not a function" err

Ok, jQuery included, Owl Carousel included, checked checked checked, still doesn't work. The error kept showing up:

Uncaught TypeError: $(…).owlCarousel is not a function


What the hell is going on?

Then I realized that the jQuery library was included twice by some of the plugins. So I'm using this following technique to get rid of the error:

(function($) {
$(document).ready(function(){
$("#owl-demo").owlCarousel({
...
 });
});
}) (jQuery);