A few years ago I wrote about the "rebaptisms" of Acts 19. Rebaptism is a word that can be hard to satisfactorily define, because its use depends on one's theological convictions. Collins English Dictionary says it is a Christian word meaning "a new or second baptism."
The predominant use of the term is an attempt to describe the baptism of a person who has received a prior baptism that is considered invalid. Almost always, if not always, those who are performing "a new or second baptism" are doing so because they believe there was never a baptism in the first place! J. D. Hall puts it this way -- It’s Not “Re-Baptism”: It’s Baptism writing, "I refuse to the word ‘baptism’ in defining that ritual [i.e. pouring water on the head of an unrepentant unbeliever]." If a church believes the candidate for baptism must be a professed believer, then a water ritual with an infant does not qualify for recognition as baptism. If a church believes the mode of baptism must be immersion, then a water ritual of affusion (pouring) or ablution or dispersion (sprinkling) does not qualify.
Baptists have traditionally understood the idea of rebaptism as a misnomer we use because it is tied into the subject under discussion -- when someone thinks a person has already been baptized and then receives "a new or second baptism."
* Baptists and others who hold believer's baptism are often dubbed "Anabaptists" or "Re-Baptizers" by those who practice infant baptism.