Can we use the same certificate with different alias name in a SmKeyDatabase?
As per the documentation (Federation Security Services Guide r12.0 SP3):
Aliases in the Smkeydatabase
Aliases enable you to easily reference any single certificate or certificate and private key pair in the smkeydatabase. Every certificate or certificate/private key pair in the smkeydatabase must have a unique alias.
The Alias should be unique. But also, because SiteMinder refers to the ID of the certificate, you cannot set the same certificate twice in the same SmKeyDatabase.