Bug for duplicate phone numbers

[nus-se-script]

Steps to reproduce

1. guest add n/John Doe p/98765432 e/john@doe.com a/Street 456 r/unknown d/vegetarian t/friend
2. guest add n/MaryLim p/98765432 e/john@doe.com a/Street 456 r/unknown d/vegetarian t/friend

Expected Unable to add as there should not be 2 people with the same phone number

Actual Was able to add 2 people with the same phone number

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_{[original: nus-cs2103-AY2324S1/pe-interim#5673] [original labels: severity.Low type.FeatureFlaw]}

[p-xp]

Team's Response

There are some instances where two guests can have the same phone number (e.g. parent and child where child uses parent's phone number).

In the future, we might explore providing warnings if duplicate phone numbers are detected. However, that is NotInScope as we do not foresee many users facing, thus making it lower priority than what has already been implemented. This bug qualifies as NotInScope because the software 'fails' gracefully when duplicate phone numbers are provided, as it adds the guests without any issues (i.e. no crashes / lags).

Duplicate status (if any):

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