Android MediatorLiveData value changed being called 2 times

Question

I have a mediator live data object in a function which fetches data from firebase.

This is my function:

 internal fun checkIfExistingUser(instance: User) {
    _user.addSource(UserDao().getAll(instance.email)){
        _user.value = decomposeDataSnapshots(it)
    }

    Log.i(SplashActivity.TAG, "3. Value added properly")
}

private fun decomposeDataSnapshots(snapshot: DataSnapshot?): User? {
    var user: User? = null

    snapshot?.children?.forEach { postSnapshot ->
        postSnapshot.getValue<User>()
            ?.let { user = it }
    }

    return user.also {
        if (it == null){
            Log.i(SplashActivity.TAG, "5. User decomposed: New User")
        }else{
            Log.i(SplashActivity.TAG, "5. User decomposed: Existing User")
        }
    }
}

On my activity, i am observing my user mediator object as such:

 private fun observeUser() = viewModel.user.observe(this, Observer {
    if(it != null){
        Log.i(TAG, "6. Observed User fetched properly.")
    }
})

This function checkIfExistingUser() is called when i click on my login button. But in the logs i can see that the function decomposeDataSnapshots() is being called twice and i don't know why. See below the log results:

2020-05-20 18:38:51.677 3236-3236/com.th3pl4gu3.locky I/Splash_Activity_Debug: 3. Value added properly
2020-05-20 18:38:51.918 3236-3236/com.th3pl4gu3.locky I/Splash_Activity_Debug: 4. Decomposition Started
2020-05-20 18:38:51.918 3236-3236/com.th3pl4gu3.locky I/Splash_Activity_Debug: 5. User decomposed: New User
2020-05-20 18:38:51.918 3236-3236/com.th3pl4gu3.locky I/Splash_Activity_Debug: 4. Decomposition Started
2020-05-20 18:38:51.918 3236-3236/com.th3pl4gu3.locky I/Splash_Activity_Debug: 5. User decomposed: New User

As you can see, the number 4 and 5 is being called twice and I don't know why.

Am I using the mediator live data in the wrong way? Please help me.