how to append to a list twice?

[John Salerno]

If I want to create a list of the form [100, 99, 99, 98, 98, 97, 97...] 
(where each item is repeated twice after the first one), how might I do 
that most efficiently?

Right now I have this:

series = [100]
for x in range(10):   # just for testing
     series.append(series[-1] - 1)

But of course that only does it once, and I don't want to have to copy 
and paste the append line. Perhaps there's a better way than this.

Thanks.