S/MIME signed attributes

Hello All,

In S/MIME signed message at MS Outlook and Netscape Messanger, the
signed message has sevral signed attributes.

I have two questions related these signed attributes.

1. "contentType" Signed Attribute
    In "encapContentInfo", the type of content is determined.
"contentType" represents the same information about content type. Why is
the same information included again?

2. "signingCertificate" Signed Attribute
    In "certificates", certificate which was used to sign message is
included. But "sigingCertificate" has the same data - the certificate
used to sign message- as value. Why?
This attribute is not used by MS Outlook and Netscape Messanger. I don't
know what the usage of this attribute is.

Thank you.

Chung