Could you give an example?
Do you mean something like
> [ manager.name for manager in set([ person.manager for person in employees ])]
?
I assume with something like set() or unique() you need to create the intermediate iterable anyway, but without it I have trouble finding an example where doing a single list comprehension wouldn't suffice.
map or filter are much easier to read for complex data manipulations and as a bonus, their composition rules make it easy to increase performance. For example, if you see two map functions together, you can wrap them in a compose and only map over your elements once. This isn't as immediately obvious when you're using comprehensions.