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

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 {

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 {
            guard !exists else {
                print("already exists")
            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