Can I use the sed command to replace multiple empty line with one empty line?

Question

Give this a try:

sed '/^$/N;/^\n$/D' inputfile

Explanation:

/^$/N – match an empty line and append it to pattern space.
; – command delimiter, allows multiple commands on one line, can be used instead of separating commands into multiple -e clauses for versions of sed that support it.
/^\n$/D – if the pattern space contains only a newline in addition to the one at the end of the pattern space, in other words a sequence of more than one newline, then delete the first newline (more generally, the beginning of pattern space up to and including the first included newline)