Find duplicate PTRs in a zone file

From ben.goodacre.name/tech

For any RFC compliant reverse zone file including BIND / named, where the first field is the last octet:

for i in `egrep '^[0-9]{1,3}' reverse.rev | awk '{print $1}' | sort -n | uniq -d` ; do egrep ^${i} reverse.rev;done

As awk is used it will still work if spaces or tabs are used. Please feel free to contact me if you find an instance where it does not work.