The odds that alien life would reinvent DNA are basically nil. If we find DNA based life elsewhere that is a slam-dunk for panspermia.
Are there many varieties of self-replicating molecules? (which I understand is needed to start life) Maaaaybe there's only so many ways that this can get started and RNA is the "only" way to do it. In a similar way that we have a hard time imagining non-carbon life since no other element behaves in the ways that carbon does