I would do this in two steps.. one to eliminate duplicates, and one to create the array:
var obj = {};
$('a.defined').each(function() {
obj[this.text] = this.title;
});
var arr = [];
for (var prop in obj) {
if (obj.hasOwnProperty(prop))
arr.push({key: prop, value: obj[prop]});
};
console.log(arr);
If the object is sufficient and you don't really need an array, you could stop after the object is created.