By the same token, when you see a wrist watch, you don't know how many other wrist watches there are out there. That wrist watch could be unique, or could be one of many.
Secularly, I take SES1066's stance. As a Christian, however, encountering intelligent aliens who are not related to humans genetically, but who are fallible, throws up a lot of difficult questions. Can these aliens be saved if they are not kinsmen of Adam or Christ? Wouldn't that be unfair and cruel if not, unless they just wink out of existence at death?