I have seen many of Catholics have their baptismal name same as that of their grandparents. Why they do so? Is it given in bible?
In Italy, it is tradition to be named after your grandparents. Firstborn children are named after their paternal grandparents and second-born children are named after their maternal grandparents. This is more of a cultural tradition than a religious tradition, and may be common in multiple European countries.
Why do they have same baptismal name?
There is no biblical references for this custom within a family. It is simply a private tradition within some Catholic families.
This seems to be the case of some family tradition, perhaps due to a particular devotion to a particular saint within a family. It may also be a family tradition of some saint related to the geographical region of where a family is living or where a family is historically from. A third possible reason for this could be that it is simply a family custom going back many generations. This family custom exists within families of Italian descent. This final possibility may in fact be the most popular custom why this happens occasionally within a family tradition. Other reasons may certainly exist.