They don't "rebaptize," they "conditionally rebaptize."
There are three things necessary to make a sacrament valid:
Form, matter, and intention.
Examples of an invalid baptism because of form:
"I baptize you in the name of the creator, the redeemer and the sanctifier."
The Novus Ordo has done, and is continuing to do, this invalid form in some places.
A baby is near death, and the priest, instead of sprinkling water on the head, uses a cotton ball with water and signs the sign of the cross over the chest of the baby and says the words of baptism.
This is invalid form, because the water is not being sprinkled over the head is necessary to confer baptism on a baby.
(I know a relative that had a sister with a baby in this situation, and this is what the novus ordo priest did.)
Example of invalid baptism because of matter:
The priest uses holy water, or soda, or juice, or some other liquid, rather than regular, natural water, for baptism.
Example of an invalid intention of the priest.
Priest holds the baby up and says, "WELCOME THIS BABY TO THE COMMUNITY."
(This happened with my sister.)
The priest has to intend to do what the Church does, which is take away original sin, leaving the indelible mark of baptism on the child/person. If they don't do that, then it's invalid intention. It's not "welcoming someone into the community." The intention is to remove original sin.
This is a sketchy area to deal with. We don't know the mind of the priest, unless he expresses it in the above way, or some other similar heretical fashion.
So if you know (or even suspect) that one of these things has happened in your baptism, best thing to do is to get conditionally rebaptized. Meaning, if your first baptism wasn't valid because of suspected improper form, matter, or intention (the first two being rather obvious, the last being hard to call without knowing the priest's state of mind) then go to your priest and have it done the right way.