diff --git a/droneapi/lib/__init__.py b/droneapi/lib/__init__.py
index bcb7a37a6..87d7fa47d 100644
--- a/droneapi/lib/__init__.py
+++ b/droneapi/lib/__init__.py
@@ -351,7 +351,8 @@ def location_callback(location):
         if l is None:
             l = []
             self.__observers[attr_name] = l
-        l.append(observer)
+        if not observer in l:
+            l.append(observer)
 
     def remove_attribute_observer(self, attr_name, observer):
         """
diff --git a/tests/sitl/test_110.py b/tests/sitl/test_110.py
new file mode 100644
index 000000000..4e220879d
--- /dev/null
+++ b/tests/sitl/test_110.py
@@ -0,0 +1,56 @@
+from droneapi.lib import VehicleMode
+from pymavlink import mavutil
+import time
+import sys
+import os
+from testlib import assert_equals
+
+def test_110(local_connect):
+    api = local_connect()
+    v = api.get_vehicles()[0]
+    
+    # Change the vehicle into STABILIZE mode
+    v.mode = VehicleMode("STABILIZE")
+
+    # NOTE wait crudely for ACK on mode update
+    time.sleep(3)
+
+    # Define example callback for mode
+    def armed_callback(attribute):
+        armed_callback.called += 1
+    armed_callback.called = 0
+
+    # When the same (event, callback) pair is passed to add_attribute_observer,
+    # only one instance of the observer callback should be added.
+    v.add_attribute_observer('armed', armed_callback)
+    v.add_attribute_observer('armed', armed_callback)
+    v.add_attribute_observer('armed', armed_callback)
+    v.add_attribute_observer('armed', armed_callback)
+    v.add_attribute_observer('armed', armed_callback)
+
+    # Disarm and see update.
+    v.armed = False
+    v.flush()
+    # Wait for ACK.
+    time.sleep(3)
+
+    # Ensure the callback was called.
+    assert armed_callback.called > 0, "Callback should have been called."
+
+    # Rmove all observers. The first call should remove all listeners
+    # we've added; the second call should be ignored and not throw.
+    # NOTE: We test if armed_callback were treating adding each additional callback
+    # and remove_attribute_observer were removing them one at a time; in this
+    # case, there would be three callbacks still attached.
+    v.remove_attribute_observer('armed', armed_callback)
+    v.remove_attribute_observer('armed', armed_callback)
+    callcount = armed_callback.called
+
+    # Re-arm and see update.
+    v.armed = True
+    v.flush()
+    # Wait for ack
+    time.sleep(3)
+
+    # Ensure the callback was called zero times.
+    assert_equals(armed_callback.called, callcount, "Callback should not have been called once removed.")