Prevent adding the same dependency twice.

TodoList.py

@@ -156,17 +156,18 @@ class TodoList(object):
 
             return '%d' % new_id
 
-        from_todo = self.todo(p_number1)
-        to_todo = self.todo(p_number2)
-
-        dep_id = None
-        if from_todo.has_tag('id'):
-            dep_id = from_todo.tag_value('id')
-        else:
-            dep_id = find_next_id()
-
-        to_todo.add_tag('p', dep_id)
-        self._depgraph.add_edge(p_number1, p_number2, int(dep_id))
+        if not self._depgraph.has_edge(p_number1, p_number2):
+            from_todo = self.todo(p_number1)
+            to_todo = self.todo(p_number2)
+
+            dep_id = None
+            if from_todo.has_tag('id'):
+                dep_id = from_todo.tag_value('id')
+            else:
+                dep_id = find_next_id()
+
+            to_todo.add_tag('p', dep_id)
+            self._depgraph.add_edge(p_number1, p_number2, int(dep_id))
 
     def remove_dependency(self, p_number1, p_number2):
         """ Removes a dependency between two todos. """

test/TodoListTest.py

@@ -174,3 +174,9 @@ class TodoListDependencyTester(unittest.TestCase):
         children = self.todolist.children(1)
         self.assertEqual([todo.source() for todo in children], \
             ['Bar p:1'])
+
+    def test_add_double_dep(self):
+        self.todolist.add_dependency(1, 2)
+
+        self.assertEqual(self.todolist.todo(1).source(), 'Foo id:1')
+        self.assertEqual(self.todolist.todo(2).source(), 'Bar p:1')