Check for duplicates in array with randomly generated values

I'm working on exercism question and am stuck on one of the jasmine-node based tests, which says that I should be able to generate 10000 random names without any clashes (e.g. 2 randomly generated names match). This is the test:

it('there can be lots of robots with different names each', function() {

    var i,
    numRobots = 10000,
    usedNames = {};

    for (i = 0; i < numRobots; i++) {
      var newRobot = new Robot();
      usedNames[newRobot.name] = true;
    }

    expect(Object.keys(usedNames).length).toEqual(numRobots);

});

What I think I need to do is:

1. Create an array to hold all the names (robotNames),
2. Each time a name is generated, check if it exists in the array,
3. If it does, generate another name,
4. If it doesn't, add it to the array.

And here is my code so far...

"use strict";

var robotNames = [];
var name;

var Robot = function() {
    this.name = this.generateName();
};

Robot.prototype.generateName = function() {

    var letters = "";
    var alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";

    var numbers = "";
    var digits = "0123456789";

    // generate random characters for robot name...

    for( var i=0; i < 2; i++ ) {
        letters += alphabet.charAt(Math.floor(Math.random() *    alphabet.length));
    };

    for( var i=0; i < 3; i++ ) {
        numbers += digits.charAt(Math.floor(Math.random() * digits.length));
    };

    name = letters+numbers;

    // Loop through array to check for duplicates

    for(var i = 0; i < robotNames.length; i++) {
        if (name == robotNames[i]) {
            this.generateName();
            return;
        } else {
            robotNames.push(name);
        }
    }

    return name;
};

Robot.prototype.reset = function() {
     this.name = this.generateName();
};

module.exports = Robot;

The test fails with an error message: "Expected 9924 to equal 10000."

The '9924' number is slightly different each time I run the test. I'm thinking this means the generateName function is eventually generating 2 matching random names. It seems as though my loop for checking duplicates is not being run and I'm not sure why.

I have tried a couple of different versions of the loop but with no success. So my questions is a) is my approach correct and there is something wrong with the syntax of my loop? or b) have I got the wrong idea about how to check for duplicates here?

Any pointers appreciated, thanks.