How can I double the newlines in an output stream

sed G

is a well known one-liner for that.

Performance-wise, the most effective with the standard Unix tool chest would probably be:

paste -d '\n' - /dev/null

If you don't want to add an empty line after the last line:

sed '$!G'

To add the empty lines before the input lines:

paste -d '\n' /dev/null -

Or:

sed 'i\
/'

using pr: Paginating Files

pr -t -d file.txt

output:

a

b

c

d

e

using awk:

awk '{printf("%s\n\n",$0)}' file.txt

With awk you can define the Output Record Separator as double new line:

awk -v ORS="\n\n" 1 file

Then, 1 performs the default awk action: {print $0}, that is, print the current line.