Add command duplicate checking

[nus-se-bot]

Seems like the app registers different capitalisations as different people ("Doe" vs "doe"). But I think it would be better if it was not case sensitive instead

Steps to replicate

1. add n/John Doe p/93456789 sch/Temasek Jc l/j2 a/135 Rivervale Link
2. add n/John doe p/93456789 sch/Temasek Jc l/j2 a/135 Rivervale Link

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_{[original: nus-cs2103-AY2122S1/pe-interim#5644] [original labels: type.FeatureFlaw severity.Medium]}

[nhjryan]

Team's Response

Going by the definition of low severity bug: A flaw that is unlikely to affect normal operations of the product. Appears only in very rare situations and causes a minor inconvenience only.

As a normal tutor, the adding of names is usually done once and hence this inconvenience is unlikely to be done many times. A simple workaround would also be to delete the duplicate person added.

Duplicate status (if any):

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