Duplicate records?

[Marek Kozlowski]

:-)
As I can see BIND allows duplicate A:

pikus    IN A 192.168.1.1
pikus    IN A 192.168.1.2

and PTR:

192.168.1.1.    IN PTR pikus.somedomain.com.
192.168.1.1.    IN PTR filemon.somedomain.com.

and disallows duplicate CNAMEs in the same way. For A and PTR both
records are returned. My questions are:

1. Is using duplicate A and PTR a standard (RFC...?) supported by all
named implementations?
2. Is using this duplicate A / PTR a good practice?
3. If A can be duplicated and CNAME cannot -- what's the reason for
using CNAMEs (A-s are better).

Thanks for any help,
Best regards,
Marek