Edit command allows 2 same persons to exist

[nus-pe-script]

Describe the bug
Editing the individual attributes email, phone and address will allow 2 persons with the exact same details to exist.

To Reproduce
Steps to reproduce the behavior:

1. Add these 2 persons in the list
   

2. Run
   edit 2 p/1234
edit 2 a/test
edit 2 p/fds@gmail.com

3. See error

Expected behavior
I expect the edits to be disallowed.

Additional context
When adding 2 persons with the exact same details is not allowed. So I presume that 2 persons with the exact same details is not allowed by design. However, I can edit 2 persons to have the exact same details.

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Reported by: @wm28
Severity: High

_{[original: nusCS2113-AY1819S1/pe-1#646]}