Is the amount of $2400 included on your T4 information slip? Sometimes this happens when the union due question is also answered elsewhere after being included when entering the same amount of the T4-slip. You may want to use the Find icon at the top right corner to locate union dues and verify if there is an amount and go to see the T4 and verify box 44. There should only be the amount in one of those areas and not both.
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