override: function(origclass, overrides) {
if (overrides) {
var p = origclass.prototype;
Ext.apply(p, overrides);
if (Ext.isIE && overrides.toString != origclass.toString) {
p.toString = overrides.toString;
}
}
}

Ext.override(Ext.util.MixedCollection, {
// The new add function, with the unique value check.
add: function(key, o) {
// The unique value check.
if (this.indexOf(o) > -1) return null;
//From this point, the code is the add function's original
//code.
if (arguments.length == 1) {
o = arguments[0];
key = this.getKey(o);
}
if (typeof key == "undefined" || key === null) {
this.length++;
this.items.push(o);
this.keys.push(null);
} else {
var old = this.map[key];
if (old) {
return this.replace(key, o);
}
this.length++;
this.items.push(o);
this.map[key] = o;
this.keys.push(key);
}
this.fireEvent("add", this.length - 1, o, key);
return o;
}
});

Ext.onReady(function() {
// Create our enhanced collection.
var col = new Ext.util.MixedCollection();
// Confirm that the same value cannot be added twice.
col.add("key 1", "value 1");
document.write("Original count: " + col.getCount() + "<br/>");
// Try to add a duplicate.
col.add("key 2", "value 1");
// Number of elements in the collection is still 1.
document.write("Count after trying to add a duplicate: " + col.getCount() + "<br/>");
});