you can use a hashtable to store values, with keys. the keys must be unique however but there still could be a duplicate value. You could iterate through each element in the array before adding the value/item in the collection and see if a duplicate exists. However you may find some perf issues doing it this way, since you wouldnt know how many items would be in the collection.
I'll see what else I can dig up, im not entirely sure about a collection which can only contain unique values.
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