http://sed.sourceforge.net/sed1line.txt

sed -n 'G; s/n/&&/; /^([ -~]*n).*n1/d; s/n//; h; P'

sed -n '                    # -n: by default, do not print
    G                       # Append hold space to current input line
    s/n/&&/                # Add empty line after current input line
    /^([ -~]*n).*n1/d  # If the current input line is repeated in the hold space, skip this line
                            # Otherwise, clean up for storing all input in hold space:
    s/n//                  # Remove empty line after current input line
    h                       # Copy entire pattern space back to hold space
    P                       # Print current input line'

^    # Look at beginning of line (i.e. beginning of pattern space)
(   # This starts group 1
[ -~] # Any printable character (in the C locale)
*     # Any number of times
n    # Followed by a newline
)   # End of group 1 -- it contains the current input line
.*n # Skip any amount of lines as necessary
1   # Another occurrence of the current input line, with newline and all

vvv
sed -n 'G; s/n/&&/; /^([^n]*n).*n1/d; s/n//; h; P'
                          ^^^

# The standard sed loop
my $hold = "";
while ($my pattern = <>) {
    chomp $pattern;

    $pattern = "$patternn$hold";           # G
    $pattern =~ s/(n)/$1$1/;               # s/n/&&/
    if ($pattern =~ /^([^n]*n).*n1/) {  # /…/
        next;                               # d
    }
    $pattern =~ s/n//;                     # s/n//
    $hold = $pattern;                       # h
    $pattern =~ /^([^n]*n?)/; print $1;   # P
}