Find all duplicate elements in a list of integers. Return a new list containing only the duplicate values, with each duplicate appearing exactly once in the result.
Examples:
Input: numbers = [1, 2, 3, 2, 4, 5, 4, 6]
Output: [2, 4]
Explanation: 2 and 4 each appear twice in the input list
Input: numbers = [7, 8, 9, 10]
Output: []
Explanation: No duplicates in the list, return empty list
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