Write an Apex method that takes a string as input and returns a list of characters that appear more than once in the string. The list should not contain any duplicate characters.
Examples:
Input: 'programming'
Output: ['r', 'g', 'm']
Explanation: The characters 'r', 'g', and 'm' each appear twice in the input string.
Input: 'hello'
Output: ['l']
Explanation: The character 'l' appears twice in the input string.
Input: 'world'
Output: []
Explanation: No characters are repeated in the input string.
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