diff --git a/mongoengine/base.py b/mongoengine/base.py
index fa6f825f..25c3bbd5 100644
--- a/mongoengine/base.py
+++ b/mongoengine/base.py
@@ -205,8 +205,12 @@ class BaseField(object):
     def __set__(self, instance, value):
         """Descriptor for assigning a value to a field in a document.
         """
-        instance._data[self.name] = value
-        if instance._initialised:
+        changed = False
+        if (self.name not in instance._data or
+            instance._data[self.name] != value):
+            changed = True
+            instance._data[self.name] = value
+        if changed and instance._initialised:
             instance._mark_as_changed(self.name)
 
     def error(self, message="", errors=None, field_name=None):
@@ -317,12 +321,6 @@ class ComplexBaseField(BaseField):
 
         return value
 
-    def __set__(self, instance, value):
-        """Descriptor for assigning a value to a field in a document.
-        """
-        instance._data[self.name] = value
-        instance._mark_as_changed(self.name)
-
     def to_python(self, value):
         """Convert a MongoDB-compatible type to a Python type.
         """