I'm not sure about preg, but with ereg it would be:
$str = "aaaaaaaaa\nbbbbbbbb\nccccccccc\nddddddddd\n...\nzzz";
$str2 = ereg_replace("([\n]*\n){2,2}","",$str);
echo "$str<br><br>$str2";
This translates as 'any number of anything except newline, followed by a newline, twice in a row, at the start of the string'
Hope that helps.