Example below: after adding the task "work" on 15/11/2019 23:00, my attempt to add "work" at 15/11/2019 23:30 shows an error, the task list only showing one "work" entry.
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adding "work"

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"work" is added and shown on task list

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attempting to add "work" at a different time slot. i.e. 23:30

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failed, with duplicate error message

[original: nus-cs2103-AY1920S1/pe-interim#2554]