Add check for duplicate phone number

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

@@ -82,12 +82,23 @@ public boolean hasSameEmail(Person otherPerson) {
         return otherPerson != null && otherPerson.getEmail().equals(this.getEmail());
     }
 
+    /**
+     * Returns true if two contacts share the same phone number.
+     * Checks for duplicates in phone numbers in the contact list.
+     */
+    public boolean hasSamePhoneNumber(Person otherPerson) {
+        return otherPerson != null && otherPerson.getPhone().equals(this.getPhone());
+    }
+
     /**
      * 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);
+        return this.isSamePerson(otherPerson)
+                || this.hasSameEmail(otherPerson)
+                || this.hasSamePhoneNumber(otherPerson);
     }
 
     /**