Implement duplicate contact handling

[shardhrv]

Implement duplicate contact handling based on the following criteria:

Rules for Duplicate Detection:

• Two contacts are considered duplicates if any one of the following criteria holds true
  • They have the exact same phone number
  • They have the exact same email
• Having the same name alone is not enough to be classified as a duplicate.

Application Reaction to Duplicates:

Duplicate entries will be rejected with the message: "Duplicate contact found: A contact with the same email or phone number already exists."

Rationale:

This prevents the creation of multiple records of same phone number or email

[cshao02]

Seems like duplicate handling is implemented based on comparing all fields: 2 people are the same if and only if they are identical in every field.