gistfile1.js

Array.prototype.duplicates = function ( ) {
    return this.filter ( function ( x , y , k ) {
        return y !== k.lastIndexOf ( x );
    });
};

Array.prototype.hasDuplicates = function ( ) {
    var me = this;
    return me.some ( function ( idx ) {
        return me.indexOf( idx ) !== me.lastIndexOf ( idx );
    });
};

var arr = [ 'a' , 'b' , 'c' , 'd' , 'a' , 'b' , 'c' , 'e' ];
console.group( 'Array Duplicada' );
console.log( arr.hasDuplicates() );
if( arr.hasDuplicates() ){
   console.log( arr.duplicates() );
}
console.groupEnd( 'Array Duplicada' );

console.group( 'Array Normal' );
var arr2 = [ 'a' , 'b' , 'c' , 'e' ];
console.log( arr2.hasDuplicates() );
if( arr2.hasDuplicates() ){
   console.log( arr2.duplicates() );
}
console.groupEnd( 'Array Normal' );