Single and Double Click on Arduino Multiplexed Buttons
In am not familiar with Arduino and the libraries but it doesn't really depend on that anyway.

You will at least require a timer of some sort that runs in the background. This timer "expires" after the desired double-click speed (time between two clicks). The timer is reset after each button press.

Then perform some checks like this (pseudo-code):

...
if (buttonHasBeenPressed) {
    if (timerIsExpired || (currentButton != previousButton)) {
        Print("Button has been clicked");
    } else {
        Print("Button has been DOUBLE clicked");             
    }

    ResetTimer(); 
}
...


This is a quick shot, but should give you an idea how to approach it. 

EDIT: I just realized in this solution, a double click will always be preceded by a single click. This is probably not what you desire, so you have to actually do some additional checks.