$ cat sd
sed -n '/^$/!{
h
}
/^$/{
N
/^\n$/ {
x
p
q
}
}' fl
$
$ cat fl
someword someword:1
new-word new-word abcd
someword someword:10
new-word new-word abcd
thisis whatIneed:3


someword someword:5
new-word new-word abcd
$
$ sd
thisis whatIneed:3