Assign roles that already exist in database on user registration post request - spring-boot

Question

I m trying to create a Spring Boot application that a user can select type of role on registration e.g patient/doctor
The roles are created automatically in the database on application start
I m using Postman to make registration request but when I pass the role it is duplicated in the database
Any advice would be much appreciated!
Here is the method i use to create a user
public User createUser (RegistrationRequest registrationRequest) {
User user = new User();
user.setFirstname(registrationRequest.getFirstname());
user.setLastname(registrationRequest.getLastname());
user.setEmail(registrationRequest.getEmail());
user.setUsername(registrationRequest.getUsername());
user.setPhone(registrationRequest.getPhone());
user.setAddress(registrationRequest.getAddress());
user.setPassword(passwordEncoder.encode(registrationRequest.getPassword()));
user.setCreatedAt(Instant.from(Instant.now()));
user.setIsEnabled(false);
user.setRoles(registrationRequest.getRoles());
return user;
and here is the json request
{
"firstname":"Mary",
"lastname":"K",
"email":"example#gmail.com",
"username":"rain",
"password":"123",
"phone":"2848392",
"address":
{
"city": "Burdwan",
"region": "Paschimbanga",
"street": "gwg",
"zipcode": "713102"
},
"roles":[
{
"name":"DOCTOR_ROLE"
} ]
}