Update EditCommand.java and Person.java

Include new method isSameDateTime to check if other person has the
same datetime

src/main/java/seedu/address/logic/commands/EditCommand.java

@@ -99,7 +99,7 @@ public CommandResult execute(Model model) throws CommandException {
             throw new CommandException(MESSAGE_DUPLICATE_PERSON);
         }
 
-        if (model.hasDateTime(editedPerson)) {
+        if (!personToEdit.isSameDateTime(editedPerson) && model.hasDateTime(editedPerson)) {
             throw new CommandException(MESSAGE_DUPLICATE_DATETIME);
         }
 

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

@@ -103,6 +103,18 @@ public boolean isSamePerson(Person otherPerson) {
                 && otherPerson.getName().equals(getName());
     }
 
+    /**
+     * Returns true if both persons have the same set of datetime.
+     */
+    public boolean isSameDateTime(Person otherPerson) {
+        if (otherPerson == this) {
+            return true;
+        }
+
+        return otherPerson != null
+                && otherPerson.getDateTimes().equals(getDateTimes());
+    }
+
     /**
      * Returns true if both persons have the same identity and data fields.
      * This defines a stronger notion of equality between two persons.