Write a method that finds duplicate values in a list of integers. The method should return a list containing each duplicate value exactly once, in the order they were first encountered as duplicates.

If no duplicates are found, return an empty list. Handle null input by returning null.

Examples:

List<Integer> duplicates = findDuplicateValues(new List<Integer>{1, 2, 3, 2, 4, 3, 5});
// Returns: [2, 3]

List<Integer> noDuplicates = findDuplicateValues(new List<Integer>{1, 2, 3, 4, 5});
// Returns: []

List<Integer> nullInput = findDuplicateValues(null);
// Returns: null
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