Multiple names allowed

[nus-pe-script]

Describe the bug
The same person can be added to the member list when I type in the same person's name in a separate command but in different casing.

To Reproduce
Steps to reproduce the behavior:

1. Type: addmember kang ting and press "ENTER".
2. Type addmember Kang Ting and press "ENTER".
3. Type listmembers and press "ENTER".
4. See error
   

Expected behavior
Message should output "This member already exists in the address book", as it happens when I type the same person's name in the same letter casing. However, when I type the same person's name of different casing (see the steps above), the expected behavior does not show.

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Reported by: @jylee-git
Severity: High

_{[original: nusCS2113-AY1819S1/pe-1#284]}

[kangmingtay]

I'll add another field called phone number so there's more differentiation between 2 members with the same name