Using Firestore DB with swift 4.0, saving to the database creates a random character document each time I authenticate

Question

I am creating an iPhone app in swift 4.0. I'm connecting over to Firestore, which currently works. The problem that I'm having is that a random character is document is created each time a user is authenticated. I want to change this so the email address is the registered document in the DB so the next time I login with the same credentials its uses the same document.

let databaseRef = Database.database().reference(fromURL: “*****************”)
let storage = Storage.storage().reference()
let db = Firestore.firestore()
var ref: DocumentReference? = nil
var handle: AuthStateDidChangeListenerHandle?
var checkBtnTitle:Bool = true


    override func viewDidLoad() {
    super.viewDidLoad()
        if Auth.auth().currentUser != nil {
            let currentUser = Auth.auth().currentUser
            btnLogOut.title = "LogOut"
            btnLogin.title = "Login"
            var ref: DocumentReference? = nil
            ref = db.collection("Solicitor").addDocument(data: [
                "userID": currentUser?.email! as Any,
                "Name": currentUser?.displayName as Any
            ]) { err in
                if let err = err {
                    print("Error adding document: \(err)")
                } else {
                    print("Document added with ID: \(ref!.documentID)")
                }
            }
        } else {
            btnLogOut.title = "Login"
            btnLogin.title = ""
        }

      // google signin
       GIDSignIn.sharedInstance()?.uiDelegate = self