How to remove duplicate items from a list using list comprehension?

Use keys on a dict constructed with values in a as its keys.

b = dict([(i, 1) for i in a]).keys()

Or use a set:

b = [i for i in set(a)]

If you don't mind using a different technique than list comprehension you can use a set for that:

>>> a = [1, 2, 3, 3, 5, 9, 6, 2, 8, 5, 2, 3, 5, 7, 3, 5, 8]
>>> b = list(set(a))
>>> print b
[1, 2, 3, 5, 6, 7, 8, 9]

It's producing an identical list as b contains no elements at run-time. What you'd want it this:

>>> a = [1, 2, 3, 3, 5, 9, 6, 2, 8, 5, 2, 3, 5, 7, 3, 5, 8]
>>> b = []
>>> [b.append(item) for item in a if item not in b]
[None, None, None, None, None, None, None, None]
>>> b
[1, 2, 3, 5, 9, 6, 8, 7]