`object in list` behaves different from `object in dict`?

I've got an iterator with some objects in it and I wanted to create a collection of uniqueUsers in which I only list every user once. So playing around a bit I tried it with both a list and a dict:

>>> for m in ms: print m.to_user  # let's first look what's inside ms
...
Pete Kramer
Pete Kramer
Pete Kramer
>>> 
>>> uniqueUsers = []  # Create an empty list
>>> for m in ms:
...     if m.to_user not in uniqueUsers:
...         uniqueUsers.append(m.to_user)
...
>>> uniqueUsers
[Pete Kramer]  # This is what I would expect
>>> 
>>> uniqueUsers = {}  # Now let's create a dict
>>> for m in ms:
...     if m.to_user not in uniqueUsers:
...         uniqueUsers[m.to_user] = 1
...
>>> uniqueUsers
{Pete Kramer: 1, Pete Kramer: 1, Pete Kramer: 1}

So I tested it by converting the dict to a list when doing the if statement, and that works as I would expect it to:

>>> uniqueUsers = {}
>>> for m in ms:
...     if m.to_user not in list(uniqueUsers):
...         uniqueUsers[m.to_user] = 1
...
>>> uniqueUsers
{Pete Kramer: 1}

and I can get a similar result by testing against uniqueUsers.keys().

The thing is that I don't understand why this difference occurs. I always thought that if you do if object in dict, it simply creates a list of the dicts keys and tests agains that, but that's obviously not the case.

Can anybody explain how object in dict internally works and why it doesn't behave similar to object in list (as I would expect it to)?