Implement duplication check when adding tasks

There is a possibility where users add repeated tasks with the same
description. This check is to prevent users from accidentally adding
duplicate tasks.

Let's
* Add duplication check in TaskList
* Check if tasks are equal based on their description

src/main/java/tick/storage/TaskList.java

@@ -46,8 +46,12 @@ public TaskList(Task... tasks) {
      *
      * @param task Task to be added.
      */
-    public void addTask(Task task) {
+    public void addTask(Task task) throws TickException {
         assert task != null : "Task cannot be null.";
+        boolean isDuplicate = this.tasks.contains(task);
+        if (isDuplicate) {
+            throw new TickException("Task is already in the list!");
+        }
         this.tasks.add(task);
     }
 

src/main/java/tick/tasks/Task.java

@@ -76,4 +76,23 @@ public void markAsUndone() {
     public String toString() {
         return String.format("[%s] %s", this.getStatusIcon(), this.description);
     }
+
+    /**
+     * Checks if the task is equal to another object.
+     * Two tasks are considered equal if they have the same description.
+     *
+     * @param obj The object to compare with.
+     * @return True if the tasks are equal; false otherwise.
+     */
+    @Override
+    public boolean equals(Object obj) {
+        if (obj instanceof Task) {
+            Task other = (Task) obj;
+            if (this.description == null || other.description == null) {
+                return false;
+            }
+            return this.description.equals(other.description);
+        }
+        return false;
+    }
 }

src/test/java/tick/storage/TaskListTest.java

@@ -27,7 +27,7 @@ public void markTaskAsUndoneTest() throws TickException {
     }
 
     @Test
-    public void addTaskTest() {
+    public void addTaskTest() throws TickException {
         TaskList taskList = new TaskList();
         taskList.addTask(new ToDo("homework"));
         assertEquals(1, taskList.getSize());