title: Finding duplicates in an NSMutableArray
body: <p>I have a class (colorClass) which holds 2 NSStrings (idNumber and favoriteColor).  There is an NSMutableArray (arrayColor) that holds over 50,000 colorClass objects.  What is the fastest way to find all duplicate idNumbers from all colorClass objects and return them in an array?  Right now I'm using 1 for loop that goes copies arrayColor, then filters the copied array using an NSPredicate.  This takes over 5 minutes to sort the array.  How can this be done more efficiently?</p>