Confirmed in Opera 21 and Chrome 34.0.1847.131 that having a <track> with the "default" attribute results in captions being displayed by the browser's default HTML5 player. If users are also displaying the Able Player captions, this results in two sets of captions being displayed on top of each other.

This is NOT a problem in Safari 6.1.2 (Opera, Chrome & Safari were all tested on Mac OS X). It is also not a problem in IE11 (Windows 8.1)

Two possible solutions might be:

1. Do whatever we need to do with the default attribute, then remove it from the DOM (default is not currently supported, but that's another issue).
2. Use CSS to hide the browser's captions. Not sure how to do this, or if it's even possible.