Life on another planet of any sort is an exciting scientific development regardless of how it happened. However, even if definitely proof of life is found in the Solar System, one should resist leaping to the conclusion that it is a separate "branch" of life because it's reasonable to think that it could still be from the same abiogenesis event.
I use "the same source" carefully, because it could be independent events of galaxy-level "panspermia" spreading. It can also be the case that Earth provided the source of panspermia... or, if you want to get a lot more exotic, that life started on Mars and spread to Earth through hitchhiking. We'd need to get it into a lab to see if it is truly from a separate abiogenesis event.
Fortunately, it wouldn't necessarily take long. Earth life seems to have many, many, many highly-conserved characteristics to it that show that it is all comes from a single source here, but that there is no reason to believe those are the only valid choices that life could have made. The odds of another abiogenesis event being able to pass itself off as the some one we came from is negligible. It is most likely that the life would be so immediately and completely different in many ways that it would be completely clear.