2 Domain Mapping Questions

I have two quick Domain Mapping questions.

1. From an SEO standpoint what happens if I add domain mapping

I already have some inbound links to that particular url? If someone links to the "old" blog url will it automatically forward to the new domain that is now mapped to it?

2. I understand it is possible to map more than one domain to a blog? Again, from an SEO standpoint how does this work? Both domains would share the same content but which domain gets to be indexed by Google?

Thank you.