Producing multiple items in a list comprehension

[Joel Koltner]

Is there an easy way to get a list comprehension to produce a flat list of, 
say, [x,2*x] for each input argument?

E.g., I'd like to do something like:

[ [x,2*x] for x in range(4) ]

...and receive

[ 0,0,1,2,2,4,3,6]

...but of course you really get a list of lists:

[[0, 0], [1, 2], [2, 4], [3, 6]]

I'm aware I can use any of the standard "flatten" bits of code to turn this 
back into what I want, but I was hoping there's some way to avoid the "lists 
of lists" generation in the first place?

A slightly similar problem: If I want to "merge," say, list1=[1,2,3] with 
list2=[4,5,6] to obtain [1,4,2,5,3,6], is there some clever way with "zip" to 
do so?

Thanks,
---Joel