Angular ng-repeat Error “Duplicates in a repeater are not allowed.”

I am defining a custom filter like so:

<div class="idea item" ng-repeat="item in items" isoatom>    
    <div class="section comment clearfix" ng-repeat="comment in item.comments | range:1:2">
        ....
    </div>
</div>

As you can see the ng-repeat where the filter is being used is nested within another ng-repeat

The filter is defined like this:

myapp.filter('range', function() {
    return function(input, min, max) {
        min = parseInt(min); //Make string input int
        max = parseInt(max);
        for (var i=min; i<max; i++)
            input.push(i);
        return input;
    };
});

I’m getting:

Error: Duplicates in a repeater are not allowed. Repeater: comment in item.comments | range:1:2 ngRepeatAction@https://ajax.googleapis.com/ajax/libs/angularjs/1.1.4/an