python generating sequence

Your solution doesn't work because it produces a list of tuples.

What you are looking for is a way to flatten the list you produce - the simplest way to do this is itertools.chain.from_iterable():

>>> list(itertools.chain.from_iterable([4*x]*2 for x in range(1, 5)))
[4, 4, 8, 8, 12, 12, 16, 16]

If you intended to have the last value only once, as in your example, simply slice the end of the list (seq[:-1]).

As a note, rather than repeating your value for each repetition you want, you can use a list and multiply it up (providing the values are immutable, or you don't mind them being the same object). An alternative is itertools.repeat().

A variation of @shantanoo's answer:

[ 4 * i for s in itertools.izip(*itertools.tee(range(1,5))) for i in s]

Output

[4, 4, 8, 8, 12, 12, 16, 16]

Really it was just an excuse to use itertools.tee :D

Sometimes a simple generator is all it takes:

def doubler_almost(seq):
    """
    Yield elements from a sequence twice until the last element
    which is only yielded once
    """
    iseq = iter(seq)
    y = next(iseq)
    yield y
    for x in iseq:
        yield y
        yield x
        y = x

print list(doubler_almost(range(4,20,4)))

-- Although this generator could definitely benefit from a better name...

The thing you get from this answer that you don't get from the others is that you don't need to convert to a list in order to slice off the last element -- It can be evaluated completely lazily. It also can be passed a generator and it still works (although the answer by Lattyware still works in that case as well -- but the zip answers don't unless you use itertools.tee first).

Another way:

>>> seq = [4, 8, 12, 16]
>>> seq = [i for s in zip(seq,seq) for i in s]
>>> seq
[4, 4, 8, 8, 12, 12, 16, 16]

In case of seq as iter,

>>> seq = [4, 8, 12, 16]
>>> seq = [i for s in zip(seq,seq.copy()) for i in s]
>>> seq
[4, 4, 8, 8, 12, 12, 16, 16]

Thanks @mgilson for the iter feedback.