In Python, what is the fastest algorithm for removing duplicates from a list so that all elements are unique *while preserving order*?

For example:

>>> x = [1, 1, 2, 'a', 'a', 3]
>>> unique(x)
[1, 2, 'a', 3]

Assume list elements are hashable.

Clarification: The result should keep the first duplicate in the list. For example, [1, 2, 3, 2, 3, 1] becomes [1, 2, 3].