Only left handle can be dragged, if they have the same value

On a range slider only the position of the left handle can be changed, if both handles are on the same position.

It's better if you change the code at line 284ff

if( o.range === true && this.values(1) === o.min ) {
  index += 1;
  closestHandle = $( this.handles[index] );
}

to this:

if (o.range === true && this.values(0) === this.values(1) && this.values(1) != o.max) {
  if (this.values(1) === o.min || this._valueFromMouse(position) > this.values(1)) {
    index = 1;
    closestHandle = $(this.handles[index]);
  }
}

and change the _normValueFromMouse function to this:

_normValueFromMouse: function( position ) {
  return this._trimAlignValue( this._valueFromMouse(position) );
},

_valueFromMouse: function( position ) {
  var pixelTotal,
    pixelMouse,
    percentMouse,
    valueTotal,
    valueMouse;

  ''(... code from original _normValueFromMouse function ...)''

  valueTotal = this._valueMax() - this._valueMin();
  valueMouse = this._valueMin() + percentMouse * valueTotal;

  return valueMouse;
},