How do you know whether or not your ancestors accepted your vicarious baptism for the dead? How do you know which of your ancestors missed out on the opportunity for salvation in the first place?
The answer to both your questions is simple: we don't know! Vicarious baptism for the dead does not force those who have died to accept Christ's restored gospel. Christ himself explained that baptism is required for entrance into God's kingdom (John 3:5), but baptism is obviously not the only requirement. One must also chose to accept Christ. The dead who are baptized vicariously are by no means "guaranteed salvation." They must also accept the baptism and the Christ who instigated that baptism. Some will accept the baptism performed in their behalf, but others will not. We do not know who of our kindred dead have accepted Christ and who have not; I suppose we'll only know in the next life.
Likewise, it is in theory possible that someone who has already accepted the restored gospel and been baptized might be baptized vicariously. In such a case the second baptism, though unnecessary, will certainly do no harm.
The answer to both your questions is simple: we don't know! Vicarious baptism for the dead does not force those who have died to accept Christ's restored gospel. Christ himself explained that baptism is required for entrance into God's kingdom (John 3:5), but baptism is obviously not the only requirement. One must also chose to accept Christ. The dead who are baptized vicariously are by no means "guaranteed salvation." They must also accept the baptism and the Christ who instigated that baptism. Some will accept the baptism performed in their behalf, but others will not. We do not know who of our kindred dead have accepted Christ and who have not; I suppose we'll only know in the next life.
Likewise, it is in theory possible that someone who has already accepted the restored gospel and been baptized might be baptized vicariously. In such a case the second baptism, though unnecessary, will certainly do no harm.