The serialized array indices of the duplicate-key rows that Unity could not merge into the live dictionary.
When the same key appears more than once in the serialized data, Unity keeps the first occurrence in the runtime Dictionary and lists the positions of the remaining rows here. The indices are positions in the dictionary's underlying serialized array of key/value pairs, in insertion order, and are stable across Inspector re-sorts because they reference the serialized array rather than the displayed row order. This array is empty when the dictionary has no duplicate-key rows.