Sticky Sidebar Briefly Appears Twice

This is due to the following code in your style.css:

a {
    outline: 0;
    color: #1ABC9C;
    -webkit-transition: all 0.3s ease-in-out;
    -moz-transition: all 0.3s ease-in-out;
    -o-transition: all 0.3s ease-in-out;
    transition: all 0.3s ease-in-out;
}

The transition is the cause of this; as soon as your element becomes sticky, the original element will disappear and be replaced by a clone of it. However, due to the transitions you put on the links, the disappearing will not happen instantly but take 0.3 seconds.

I suggest you either remove the transition for the links (but keep them on hover), or override the transition just for that particular sidebar.

Hope this helps!

I overlooked that when searching through style.css, did as you suggested and everything works perfectly!

Thank you very much!