Allow to change gate number to current gate number

[nus-pe-bot]

Constraints mentioned by UG does not throw error

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_{[original: nus-cs2113-AY2223S1/pe-interim#980] [original labels: severity.VeryLow type.FunctionalityBug]}

[shengiv]

Team's Response

This is an intended outcome. This is to allow the user to make sure that the gate number is correct. He can update the gate number to the same gate number as many times as he wants. This is however a documentation bug since the documentation should specify that the gate number cannot be the same as that of another flight.

Duplicate status (if any):

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