sed -e "s/pattern/replacement/g;n"

sed -e '/PATTERN/ { :inside' -e 'n;s//REPLACEMENT/;t' -e 'b inside' -e '}'

sed -e '/PATTERN/{:i' -e 'n;s//REPLACEMENT/;t' -e 'b i' -e '}'

echo -e "testnreplace_mentestntestnreplace_menreplace_menreplace_me" |
sed '/replace_me/{:a;N;/replace_me.*replace_me/!Ta;s/replace_me/replaced/2;}'