Able to add existing tasks again despite program knowing that user is trying to add an already existing task

[nus-pe-bot]

Steps to reproduce:

1. add a task that already exist

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_{[original: nusCS2113-AY1920S1/pe-interim#1337]}

[yueyuu]

Team's Response

I wanted to allow users to be able to add events that clash as sometimes some people want to go for clashing events thus I feel that this is a feature flaw.

Duplicate status (if any):

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