Traits listener bug with multiple identical items in a single list.

Traits listeners attached to list items aren't handled correctly in situations where an item appears in a list more than once. Here's an example, showing a failure of a traits listener to trigger:

``` python
from traits.api import HasStrictTraits, Int, List, on_trait_change

class Apple(HasStrictTraits):
count = Int(0)

class Bowl(HasStrictTraits):
apples = List(Apple)

@on_trait_change('apples:count')
def print_status(self, obj, name, old, new):
print "Count updated from {} to {} for {}".format(old, new, obj)

granny_smith = Apple()
bowl = Bowl(apples=[granny_smith, granny_smith])
granny_smith.count = 42 # Triggers the listener, as expected.
bowl.apples.pop()
granny_smith.count = 1729 # Listener not triggered. (Surprise!)
```