The other replies will work well - but what if you want it to stay in order?
Then here is a short perl line.
[paul at www1 lib]$ echo "foo
foo
bar
foo
bar
bing" | perl -ne 'print if ! $used{$_}++'
foo
bar
bing
And if you only want to deal with lines with ips - then you could do
cat /etc/hosts.deny | perl -ne 'print if ! /^ALL:(\S+)/ || ! $used{$1}++'
Paul