Implement duplicate email checking feature

src/main/java/seedu/address/model/person/Person.java

@@ -74,6 +74,22 @@ public boolean isSamePerson(Person otherPerson) {
                 && otherPerson.getName().equals(getName());
     }
 
+    /**
+     * Returns true if both person have the same email address.
+     * This is to check for duplicates in emails in the contact list.
+     */
+    public boolean hasSameEmail(Person otherPerson) {
+        return otherPerson != null && otherPerson.getEmail().equals(this.getEmail());
+    }
+
+    /**
+     * Returns true if two contacts are considered as duplicates.
+     * This is to avoid adding duplicate contact in the contact list.
+     */
+    public boolean hasDuplicateInfo(Person otherPerson) {
+        return this.isSamePerson(otherPerson) || this.hasSameEmail(otherPerson);
+    }
+
     /**
      * Returns true if both persons have the same identity and data fields.
      * This defines a stronger notion of equality between two persons.

src/main/java/seedu/address/model/person/UniquePersonList.java

@@ -33,7 +33,7 @@ public class UniquePersonList implements Iterable<Person> {
      */
     public boolean contains(Person toCheck) {
         requireNonNull(toCheck);
-        return internalList.stream().anyMatch(toCheck::isSamePerson);
+        return internalList.stream().anyMatch(toCheck::hasDuplicateInfo);
     }
 
     /**

src/test/java/seedu/address/logic/commands/EditCommandTest.java

@@ -5,6 +5,7 @@
 import static org.junit.jupiter.api.Assertions.assertTrue;
 import static seedu.address.logic.commands.CommandTestUtil.DESC_AMY;
 import static seedu.address.logic.commands.CommandTestUtil.DESC_BOB;
+import static seedu.address.logic.commands.CommandTestUtil.VALID_EMAIL_BOB;
 import static seedu.address.logic.commands.CommandTestUtil.VALID_NAME_BOB;
 import static seedu.address.logic.commands.CommandTestUtil.VALID_PHONE_BOB;
 import static seedu.address.logic.commands.CommandTestUtil.VALID_TAG_HUSBAND;
@@ -55,11 +56,11 @@ public void execute_someFieldsSpecifiedUnfilteredList_success() {
         Person lastPerson = model.getFilteredPersonList().get(indexLastPerson.getZeroBased());
 
         PersonBuilder personInList = new PersonBuilder(lastPerson);
-        Person editedPerson = personInList.withName(VALID_NAME_BOB).withPhone(VALID_PHONE_BOB)
-                .withTags(VALID_TAG_HUSBAND).build();
+        Person editedPerson = personInList.withName(VALID_NAME_BOB).withEmail(VALID_EMAIL_BOB)
+                .withPhone(VALID_PHONE_BOB).withTags(VALID_TAG_HUSBAND).build();
 
         EditPersonDescriptor descriptor = new EditPersonDescriptorBuilder().withName(VALID_NAME_BOB)
-                .withPhone(VALID_PHONE_BOB).withTags(VALID_TAG_HUSBAND).build();
+                .withPhone(VALID_PHONE_BOB).withTags(VALID_TAG_HUSBAND).withEmail(VALID_EMAIL_BOB).build();
         EditCommand editCommand = new EditCommand(indexLastPerson, descriptor);
 
         String expectedMessage = String.format(EditCommand.MESSAGE_EDIT_PERSON_SUCCESS, Messages.format(editedPerson));
@@ -87,9 +88,11 @@ public void execute_filteredList_success() {
         showPersonAtIndex(model, INDEX_FIRST_PERSON);
 
         Person personInFilteredList = model.getFilteredPersonList().get(INDEX_FIRST_PERSON.getZeroBased());
-        Person editedPerson = new PersonBuilder(personInFilteredList).withName(VALID_NAME_BOB).build();
+
+        Person editedPerson = new PersonBuilder(personInFilteredList)
+                .withName(VALID_NAME_BOB).withEmail(VALID_EMAIL_BOB).build();
         EditCommand editCommand = new EditCommand(INDEX_FIRST_PERSON,
-                new EditPersonDescriptorBuilder().withName(VALID_NAME_BOB).build());
+                new EditPersonDescriptorBuilder().withName(VALID_NAME_BOB).withEmail(VALID_EMAIL_BOB).build());
 
         String expectedMessage = String.format(EditCommand.MESSAGE_EDIT_PERSON_SUCCESS, Messages.format(editedPerson));
 

src/test/java/seedu/address/model/person/PersonTest.java

@@ -4,6 +4,7 @@
 import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 import static seedu.address.logic.commands.CommandTestUtil.VALID_ADDRESS_BOB;
+import static seedu.address.logic.commands.CommandTestUtil.VALID_EMAIL_AMY;
 import static seedu.address.logic.commands.CommandTestUtil.VALID_EMAIL_BOB;
 import static seedu.address.logic.commands.CommandTestUtil.VALID_NAME_BOB;
 import static seedu.address.logic.commands.CommandTestUtil.VALID_PHONE_BOB;
@@ -51,6 +52,31 @@ public void isSamePerson() {
         assertFalse(BOB.isSamePerson(editedBob));
     }
 
+    @Test
+    public void hasSameEmail() {
+        assertTrue(ALICE.hasSameEmail(ALICE));
+
+        Person editedAlice = new PersonBuilder(ALICE).withEmail(VALID_EMAIL_BOB).build();
+        assertFalse(ALICE.hasSameEmail(editedAlice));
+
+        Person editedBob = new PersonBuilder(BOB).withEmail(VALID_EMAIL_AMY).build();
+        Person aliceWithNewEmail = new PersonBuilder(ALICE).withEmail(VALID_EMAIL_AMY).build();
+        assertTrue(aliceWithNewEmail.hasSameEmail(editedBob));
+
+        assertFalse(ALICE.hasSameEmail(null));
+    }
+
+    @Test
+    public void hasDuplicateInfo() {
+        assertTrue(ALICE.hasDuplicateInfo(ALICE));
+        assertFalse(ALICE.hasDuplicateInfo(null));
+
+        Person editedBob = new PersonBuilder(BOB).withEmail(VALID_EMAIL_AMY).build();
+        Person aliceWithNewEmail = new PersonBuilder(ALICE).withEmail(VALID_EMAIL_AMY).build();
+        assertTrue(editedBob.hasDuplicateInfo(aliceWithNewEmail));
+        assertFalse(editedBob.hasDuplicateInfo(ALICE));
+    }
+
     @Test
     public void equals() {
         // same values -> returns true