I am a beginner in android development and am making a pretty simple Login-Signup app that first logs in the users with email and password and then goes on to Phone authentication to verify their phone number via OTP. Now I found that Firebase provides different user ids to the Email/password authentication and Phone authentication. But I need them both to be under one userid so that they can login their account through email or phone number.

I did read various answers and official Firebase documentation on how to link multiple auth providers using linkWithCredential and tried to implement it in my code but it always fails showing error that this email is already in use by another account.
I couldn’t follow where I need to place the linkWithCredential method.

Please guide me, I am attaching the code for the two activities.

1)LoginActivity

package com.example.bccl

import android.content.ContentValues.TAG
import android.content.Intent
import android.os.Bundle
import android.view.View
import android.widget.Toast
import androidx.activity.enableEdgeToEdge
import androidx.appcompat.app.AppCompatActivity
import androidx.core.view.ViewCompat
import android.os.Handler
import android.os.Looper
import android.text.InputType
import android.util.Log
import androidx.core.view.WindowInsetsCompat
import com.example.bccl.databinding.ActivityLoginBinding
import com.google.firebase.FirebaseException
import com.google.firebase.auth.FirebaseAuth
import com.google.firebase.auth.PhoneAuthCredential
import com.google.firebase.auth.PhoneAuthOptions
import com.google.firebase.auth.PhoneAuthProvider
import com.google.firebase.auth.ktx.auth
import com.google.firebase.firestore.ktx.firestore
import com.google.firebase.ktx.Firebase
import java.util.concurrent.TimeUnit

class LoginActivity : AppCompatActivity() {
    private val binding: ActivityLoginBinding by lazy {
        ActivityLoginBinding.inflate(layoutInflater)
    }

    private lateinit var auth: FirebaseAuth
    private var passwordShowing = false
    private val db = Firebase.firestore

    override fun onStart() {
        super.onStart()
        val currentUser = auth.currentUser
        currentUser?.let {
            Log.d(TAG,"user id is ${currentUser.uid}")
            db.collection("user").document(currentUser.uid)
                .get()
                .addOnSuccessListener {
                    if (it != null && it.data?.get("otpverified") == true) {
                        val intent = Intent(this, MainActivity::class.java)
                        startActivity(intent)
                        finish()
                    }
                }



        }
    }

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        enableEdgeToEdge()
        setContentView(binding.root)

        auth = Firebase.auth

        binding.eyepassword2.setOnClickListener {
            passwordShowing = !passwordShowing

            if (passwordShowing) {
                binding.editTextTextPassword.inputType = InputType.TYPE_TEXT_VARIATION_VISIBLE_PASSWORD
                binding.eyepassword2.setImageResource(R.drawable.eye)
            } else {
                binding.editTextTextPassword.inputType = InputType.TYPE_CLASS_TEXT or InputType.TYPE_TEXT_VARIATION_PASSWORD
                binding.eyepassword2.setImageResource(R.drawable.eye_off)
            }
            binding.editTextTextPassword.setSelection(binding.editTextTextPassword.text.length)
        }

        binding.SignUpText.setOnClickListener {
            val intent = Intent(this, SignUp::class.java)
            startActivity(intent)
            finish()
        }

        val currentUser = auth.currentUser
        if (currentUser != null) {
            Log.d(TAG,"user id is ${currentUser.uid}")
        }

        binding.loginbutton.setOnClickListener {
            val email = binding.editTextTextEmailAddress.text.toString().trim()
            val password = binding.editTextTextPassword.text.toString().trim()

            if (email.isBlank() || password.isBlank()) {
                Toast.makeText(this, "Please Fill In All The Details", Toast.LENGTH_LONG).show()
            } else if (!android.util.Patterns.EMAIL_ADDRESS.matcher(email).matches()) {
                Toast.makeText(this, "Please Enter a Valid Email", Toast.LENGTH_LONG).show()
            } else {
                binding.progressBar.visibility = View.VISIBLE
                binding.loginbutton.visibility = View.INVISIBLE
                binding.loginbutton.isEnabled = false

                auth.signInWithEmailAndPassword(email, password)
                    .addOnCompleteListener(this) { task ->
                        binding.progressBar.visibility = View.GONE
                        binding.loginbutton.visibility = View.VISIBLE
                        binding.loginbutton.isEnabled = true

                        if (task.isSuccessful) {
                            Log.d(TAG, "signInWithEmail:success")
                            val intent = Intent(this, OTPVerificationActivity::class.java)

                            val userId = auth.currentUser?.uid
                            userId?.let {
                                val ref = db.collection("user").document(it)
                                ref.get()
                                    .addOnSuccessListener { document ->
                                        if (document != null) {
                                            val phoneNumber = document.data?.get("phone").toString()

                                            val options = PhoneAuthOptions.newBuilder(auth)
                                                .setPhoneNumber("+91$phoneNumber")
                                                .setTimeout(60L, TimeUnit.SECONDS)
                                                .setActivity(this)
                                                .setCallbacks(object : PhoneAuthProvider.OnVerificationStateChangedCallbacks() {
                                                    override fun onVerificationCompleted(credential: PhoneAuthCredential) {
                                                        // Handle verification completion
                                                    }

                                                    override fun onVerificationFailed(e: FirebaseException) {
                                                        Toast.makeText(
                                                            this@LoginActivity,
                                                            e.message,
                                                            Toast.LENGTH_SHORT
                                                        ).show()
                                                    }

                                                    override fun onCodeSent(
                                                        verificationId: String,
                                                        token: PhoneAuthProvider.ForceResendingToken
                                                    ) {
                                                        intent.putExtra("verificationId", verificationId)
                                                        startActivity(intent)
                                                        finish()
                                                    }

                                                    override fun onCodeAutoRetrievalTimeOut(verificationId: String) {
                                                        // Handle code auto-retrieval timeout
                                                    }
                                                })
                                                .build()

                                            PhoneAuthProvider.verifyPhoneNumber(options)
                                        }
                                    }
                            }
                        } else {
                            Log.w(TAG, "signInWithEmail:failure", task.exception)
                            Toast.makeText(baseContext, "Invalid Credentials.", Toast.LENGTH_SHORT).show()
                        }
                    }
            }
        }

        ViewCompat.setOnApplyWindowInsetsListener(findViewById(R.id.main)) { v, insets ->
            val systemBars = insets.getInsets(WindowInsetsCompat.Type.systemBars())
            v.setPadding(systemBars.left, systemBars.top, systemBars.right, systemBars.bottom)
            insets
        }
    }
}

2. OTPVerification Activity

package com.example.bccl

import android.content.ContentValues.TAG
import android.content.Intent
import android.os.Bundle
import android.text.Editable
import android.text.TextWatcher
import android.util.Log
import android.view.View
import android.widget.Toast
import androidx.activity.enableEdgeToEdge
import androidx.appcompat.app.AppCompatActivity
import androidx.core.view.ViewCompat
import androidx.core.view.WindowInsetsCompat
import com.example.bccl.databinding.ActivityOtpverificationBinding
import com.google.firebase.auth.FirebaseAuth
import com.google.firebase.auth.FirebaseAuthInvalidCredentialsException
import com.google.firebase.auth.PhoneAuthCredential
import com.google.firebase.auth.PhoneAuthProvider
import com.google.firebase.auth.ktx.auth
import com.google.firebase.firestore.ktx.firestore
import com.google.firebase.ktx.Firebase

class OTPVerificationActivity : AppCompatActivity() {
    private val binding: ActivityOtpverificationBinding by lazy {
        ActivityOtpverificationBinding.inflate(layoutInflater)
    }

    private val db = Firebase.firestore
    private val auth = Firebase.auth
    private var realuser: String? = null
    private var hero : String? = null

    override fun onStart() {
        super.onStart()
        Log.d(TAG,"onStart() method")
        hero = auth.currentUser?.uid
        if (hero != null) {
            Log.d(TAG, "User ID :$hero")
        }

    }

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        enableEdgeToEdge()
        setContentView(binding.root)

        fetchPhoneNumber()

        val otpbackend = intent.getStringExtra("verificationId")

        binding.verifybutton.setOnClickListener {
            Log.d(TAG,"User ID :" + auth.currentUser?.uid)
            realuser= auth.currentUser?.uid
            verifyOtp(otpbackend)
        }

        ViewCompat.setOnApplyWindowInsetsListener(findViewById(R.id.main)) { v, insets ->
            val systemBars = insets.getInsets(WindowInsetsCompat.Type.systemBars())
            v.setPadding(systemBars.left, systemBars.top, systemBars.right, systemBars.bottom)
            insets
        }
        setupOtpInputs()
    }

    private fun fetchPhoneNumber() {
        val userId = FirebaseAuth.getInstance().currentUser?.uid
        userId?.let { uid ->
            val ref = db.collection("user").document(uid)
            ref.get()
                .addOnSuccessListener {
                    it?.data?.get("phone")?.toString()?.let { phoneNumber ->
                        binding.phnnumber.text = String.format("+91-%s", phoneNumber)
                        Toast.makeText(this, "Phone Number Fetched Successfully", Toast.LENGTH_SHORT).show()
                        Log.d("Firebase", "DocumentSnapshot data: ${it.data}")
                    }
                }
                .addOnFailureListener { exception ->
                    Log.e("Firebase", "Error fetching phone number", exception)
                }
        }
    }

    private fun verifyOtp(otpbackend: String?) {
        val otp = binding.editTextText1.text.toString() +
                binding.editTextText2.text.toString() +
                binding.editTextText3.text.toString() +
                binding.editTextText4.text.toString() +
                binding.editTextText5.text.toString() +
                binding.editTextText6.text.toString()

        if (otp.length != 6) {
            Toast.makeText(this, "Please Enter OTP", Toast.LENGTH_SHORT).show()
            return
        }

        if (otpbackend == null) {
            Toast.makeText(this, "Please Check Internet Connection", Toast.LENGTH_SHORT).show()
            return
        }

        binding.progressBar.visibility = View.VISIBLE
        binding.verifybutton.visibility = View.GONE

        val credential = PhoneAuthProvider.getCredential(otpbackend, otp)

        auth.currentUser?.let { currentUser ->
            currentUser.linkWithCredential(credential)
                .addOnCompleteListener(this) { task ->
                    binding.progressBar.visibility = View.GONE
                    binding.verifybutton.visibility = View.VISIBLE
                    if (task.isSuccessful) {
                        Log.d(TAG, "linkWithCredential:success")

                    } else {
                        Log.w(TAG, "linkWithCredential:failure", task.exception)
                        if (task.exception is FirebaseAuthInvalidCredentialsException) {
                            Toast.makeText(this, "Invalid OTP.", Toast.LENGTH_SHORT).show()
                        } else {
                            Toast.makeText(this, "Log-in failed.", Toast.LENGTH_SHORT).show()
                        }
                    }

                }

            signInWithPhoneAuthCredential(credential)
        }
    }

    private fun signInWithPhoneAuthCredential(credential: PhoneAuthCredential) {
        auth.signInWithCredential(credential)
            .addOnCompleteListener(this) { task ->
                binding.progressBar.visibility = View.GONE
                binding.verifybutton.visibility = View.VISIBLE

                if (task.isSuccessful) {


                    Log.d(TAG, "signInWithCredential:success,  user id : ${auth.currentUser?.uid}")

                                val currentUser = auth.currentUser

                    if (currentUser != null) {
                        markOTPVerification(currentUser.uid)
                    }

                                    // Proceed to main activity
                                    navigateToMainActivity()



                } else {
                    Log.w(TAG, "signInWithCredential:failure", task.exception)
                    if (task.exception is FirebaseAuthInvalidCredentialsException) {
                        Toast.makeText(this, "Invalid OTP.", Toast.LENGTH_SHORT).show()
                    } else {
                        Toast.makeText(this, "Log-in failed.", Toast.LENGTH_SHORT).show()
                    }
                }
            }
    }

    private fun navigateToMainActivity() {
        val intent = Intent(this, MainActivity::class.java)
        intent.flags = Intent.FLAG_ACTIVITY_NEW_TASK or Intent.FLAG_ACTIVITY_CLEAR_TASK
        startActivity(intent)
        finish()
    }

    private fun markOTPVerification(uid: String) {
        db.collection("user").document(uid)
            .update("otpverified", true)
            .addOnSuccessListener {
                Log.d(TAG, "OTP Verification marked in database successfully")
            }
            .addOnFailureListener { e ->
                Log.w(TAG, "Error marking OTP Verification", e)
                Toast.makeText(this, "Failed to mark OTP Verification", Toast.LENGTH_SHORT).show()
            }
    }

    private fun setupOtpInputs() {
        val textWatcher = object : TextWatcher {
            override fun beforeTextChanged(s: CharSequence?, start: Int, count: Int, after: Int) {}

            override fun onTextChanged(s: CharSequence?, start: Int, before: Int, count: Int) {
                if (s?.length == 1) {
                    when (currentFocus?.id) {
                        R.id.editTextText1 -> binding.editTextText2.requestFocus()
                        R.id.editTextText2 -> binding.editTextText3.requestFocus()
                        R.id.editTextText3 -> binding.editTextText4.requestFocus()
                        R.id.editTextText4 -> binding.editTextText5.requestFocus()
                        R.id.editTextText5 -> binding.editTextText6.requestFocus()
                    }
                }
            }

            override fun afterTextChanged(s: Editable?) {}
        }

        binding.editTextText1.addTextChangedListener(textWatcher)
        binding.editTextText2.addTextChangedListener(textWatcher)
        binding.editTextText3.addTextChangedListener(textWatcher)
        binding.editTextText4.addTextChangedListener(textWatcher)
        binding.editTextText5.addTextChangedListener(textWatcher)
        binding.editTextText6.addTextChangedListener(textWatcher)
    }
}