Can sed remove 'double' newline characters?

Just to remove empty lines:

sed  '/^$/d'

sed is line oriented, so thinking in terms of "2 or more of a particular byte" works except when that byte is a newline. Then you have to think of something that works for the whole line.

No need for sed. grep will do:

grep .

(that's grep, SPC, dot, that is match any line containing at least one character).

There's also:

tr -s '\n'

(squeeze any sequence of newline characters into one).

As noted by Chris, both are not equivalent because removing empty lines (like the first solution above and most other answers focus on here) is not the same as squeezing sequences of newline characters as requested in the case where the first line is empty as it only takes one leading newline character to make the first line empty.

If you wanted to keep a single blank line for any given sequence of blank lines you might do:

sed -e '/./b' -e :n -e 'N;s/\n$//;tn'