There are times where it is helpful to have multiple copies of the same record. Those times fall into two categories.
The first is when you need a better image of the record than the one that you have. Getting another image or copy make it easier to read that name or word that is giving you difficulty.
The second time is when it’s not really the same record. A birth certificate may have additional details that were not put in the birth register. A birth reference in a church record (usually mentioned in a baptism) may include details not on the actual civil copy of the birth record.
But getting another copy of something you already have because you did not know you had it is not the way to go. That was money that could have been spent on getting something you did not already have and time you could have been using to locate something else.