Keep getting 'existing email' error in Firebase - ios

Question

I am trying to setup a method to allow users to login to their Firebase accounts using Facebook. If it is a first time user creating an account everything works fine, but if a user made an account before and is trying to login in to their account is where the trouble begins. When the code
FIRAuth.auth()?.signIn(with: credential, completion: {(user, error)
runs I get the error saying that the email is already in use, even though it is the account in which they are trying to sign in for.
My entire code is here:
func handleCustomFBLogin() {
FBSDKLoginManager().logIn(withReadPermissions: ["email"], from: self) { (result, err) in
if err != nil {
print("Error loggin in is \(err)")
//self.facebookanimateIn()
} else if (result?.isCancelled)!{
print("The user cancelled loggin in ")
} else {
let credential = FIRFacebookAuthProvider.credential(withAccessToken: FBSDKAccessToken.current().tokenString)
let graphRequest : FBSDKGraphRequest = FBSDKGraphRequest(graphPath: "me", parameters: ["fields": "id, name, email"])
graphRequest.start(completionHandler: { (connection, result, error) -> Void in
if ((error) != nil) {
print("Error: \(error)")
} else {
let data:[String:AnyObject] = result as! [String: AnyObject]
let facebookName:NSString = data["name"] as! NSString
let facebookEmail = data["email"] as Any
let userId = data["id"] as! NSString
let facebookProfileUrl = "http://graph.facebook.com/\(userId)/picture?type=large"
let facebookAge = data["age_range"] as Any
let password = "needToPutRandomizedPasswordInHere" as String
FIRAuth.auth()?.createUser(withEmail: facebookEmail as! String, password: password, completion: {result, error in
if error != nil{
//user has account, they just need to sign in
FIRAuth.auth()?.signIn(with: credential, completion: {(user, error) in
if error != nil{
print(error.debugDescription)
return
}
let storyboard: UIStoryboard = UIStoryboard(name: "Main", bundle:nil)
let vc: UINavigationController = storyboard.instantiateViewController(withIdentifier: "checker") as! UINavigationController
self.present(vc, animated: true, completion: nil)
// ref.removeObserver(withHandle: handle)
})
} else {
//user does not have an account and they need to create one
guard let uid = result?.uid else{
return
}
print("user created as \(uid)")
let val = "0"
let number = (val as NSString).integerValue
let ref = FIRDatabase.database().reference()
let usersReference = ref.child("Users").child(uid)
let values = [""] as [String : Any];
usersReference.updateChildValues(values, withCompletionBlock: { (err,ref) in
if err != nil {
print(err.debugDescription)
return}})
print("Save the user successfully into Firebase database")
let storyboard: UIStoryboard = UIStoryboard(name: "Main", bundle:nil)
let vc: UINavigationController = storyboard.instantiateViewController(withIdentifier: "checker") as! UINavigationController
self.present(vc, animated: true, completion: nil)
}
})
}
})
}
}
}

Answer 1

once you have created the user, all you need is the login step
FIRAuth.auth()?.signIn(withEmail: email, password: password, completion: { (user, error) in
if error != nil {
// handle errors
} else {
// continue with login process
}
})