Once you have imported the members into the system, you can restrict them from making any duplicate accounts.
To import the members into the system you can go to :
Clients >> All Clients >> Import - To know more on Client Import - Click Here
Once the members are imported, uncheck the create account button from the Service store.
Go to Settings >> Booking Flow >> Uncheck the create account button
Once you have unchecked the create account button, the client would not be able to create any duplicate accounts further as they will only have the option to log in to their account from Service Store