Duplicate names not allowed

[breezetall]

Steps to reproduce:

1. add n/John Doe p/98765432 e/johnd@example.com a/John street, block 123, #01-01 g/johnDoee ts/Java
2. add n/John Doe p/98765432 e/j@ex.com a/block 345, #01-01 g/johnDover ts/Java

People can have the same names in real life. Different names should be allowed as long as their details are different.