IMA: do not allow the same rule to specify the same thing twice

IMA will accept rules which specify things twice and will only pay
attention to the last one.  We should reject such rules.

Signed-off-by: Eric Paris <eparis@redhat.com>
Acked-by: Mimi Zohar <zohar@us.ibm.com>
Signed-off-by: James Morris <jmorris@namei.org>