Shortest POSIX command line that eagerly removes dual newline sequences Example:

echo -ne 'a\n\nb\nc\n\n\n\nd\n\n\n' | your-command


should output the same as

echo -ne 'ab\ncd\n'


Watch out, some POSIX tools have implementations that diverge from the standard. Please consult https://pubs.opengroup.org/onlinepubs/9699919799/idx/utilities.html before submitting your solutions.
Bash, 32 bytes



awk 'BEGIN{RS="\n\n";ORS="";} 1'


Try it online!