transitionend can be fired twice

[MartinMoizard]

When controlling a paper-ripple through its API (downAction and upAction), the transitionend event can be fired more than once.

The issue can be reproduced using the following code: https://jsfiddle.net/2q3fqehz/34/

My question is the following: in upAction of paper-ripple.html, why not replacing

this.animate();

by

if (!this._animating) {  
  this.animate();  
}  

?