Check if user exists in Firebase Database and if so prevent creating an account

Using Swift
, ,
#1

would like to prevent the account creation when an email address is already registered in the database, so here is my code :

    final class DatabaseManager {
    
    static let shared = DatabaseManager()
    private let database = Database.database().reference()
    
}

// MARK: - Account Management

extension DatabaseManager {
    
    public func userExists(with email: String,
                           completion: @escaping ((Bool) -> Void)) {
        
        var safeEmail = email.replacingOccurrences(of: ".", with: "-")
        safeEmail = safeEmail.replacingOccurrences(of: "@", with: "-")
        
        database.child(safeEmail).observeSingleEvent(of: .value, with: { snapshot in
            guard snapshot.value as? String != nil else {
                completion(false)
                return
            }
            completion(true)
        })
        
    }

And then I call it like that :

  //FireBase Log In
        
        DatabaseManager.shared.userExists(with: email, completion: { [weak self] exists in
            guard let strongSelf = self else {
                return
            }
            guard !exists else {
                strongSelf.alertInvalidEmail()
                print("already exists")
                return
            }
            print("doesnt exist")
            self?.presenter(vc, animated: false, pushing: true, completion: nil) // success, present next vc
        })
    }

Here email is the email address that needs to be checked.

But the thing is that it always prints "doesnt exist" even if it does, no matter what I try. Is it because the account creation happens on the second view controller, so not at the same time as the email address check? Im really stuck thanks